The Cult of Gozer (also known as Gozer Worshippers and Gozerian Cult) (as Peter Venkman guessed and answered), was a powerful and religious organization dedicated to the worship of Gozer, the ancient god which they believed was to return to one day destroy and remake the world. The Cult was mentioned in the first Ghostbusters movie and become a major threat in Ghostbusters: The Video Game. Despite its power, the cult itself is but a tool for its leader, Ivo Shandor.
The original Gozer Worshipers, known as the "Gozerians" first appeared in Mesopotamia around 6000 B.C. and by 4000 B.C., the Cult of Gozer consisted of a large Sumerian sub-culture and was engaged in a long, protracted war with the followers of Tiamat. Eventually Gozer and its followers were defeated and Tiamat banished Gozer from this world.    
Though Gozer fell into relative obscurity, it was not forgotten over the subsequent millennia and in 1920 A.D., a new cult was founded by Ivo Shandor. Shandor was an insane genius who, during his lifetime, made all the preparations needed for his god to be summoned. The members of the cult included close to a thousand followers, whose high priests included a great many wealthy, intellectual, and powerful people in New York society and from around the world. He promised them all stations of power and glory in the new post-Gozer millennia, while secretly binding them to his own service in life and afterward.  The cult made many preparations for Gozer, including constructing a portal which would eventually bring forth the Destroyer, and four sacred places containing a Mandala to channel the spirit force, powering their god's Destructor form. The cult studied the paranormal very carefully, and had accumulated knowledge and built devices rather advanced for their time. The Cult's activities were prominent enough that they and Shandor himself are listed in Tobin's Spirit Guide. 
By 1984, during the time of the Gozer's coming, all the cult members were already dead. However, their spirits were still active on Earth, thanks to the Black Slime, a powerful substance they created from a captured Sloar. The Black Slime boosted the powers of their spirits, allowing them to operate the Mandalas and patiently wait until their god will bring new, post-Gozerian era to the Earth.
However, the mighty Gozer was defeated by the Ghostbusters, ruining the Cult's plan for a new world. The now-undead Cult planned a second coming of Gozer, which succeeded in 1991. However, the god was once again banished by the Ghostbusters. This led Ivo Shandor to reconsider his dream, activating the four Mandalas and channeling the spirit force to the Cult's secret holy ground, the Cemetery in Central Park. Disappointed in his God, Ivo Shandor planned to power himself with the spirit energy intended for Gozer and become a Destroyer himself. However, he was stopped by the Ghostbusters.
According to Tobin's Spirit Guide (GBI Supplement Version)
The first known reference to Gozer the Gozerian is a brief phrase in an Egyptian legend believed to reach back to the end of the Middle Kingdom (2040-1650 B.C.). This particular chronicle, dated to approximately 1600 B.C., warns about a powerful deity of the Hyksos known as Zuul, or the Gatekeeper, minion of Gozer. The menace of Gozer did not end with the passing of the Hyksos (the Egyptians overthrew their masters in 1567 B.C.). Though its worshipers remained quiet for more than 3000 years, remain they did.
Hawthorne Bent, another fanatical Gozer worshiper, founded the Alnwick school in 1871. The ruins of Alnwick can still be seen rising from the English moors today. Their most striking feature is the odd, pyramid-like shape of the roof of one of the buildings. One also notices the strange stone arch that is the entrance to the ruined school grounds. On one side of the arch crouches a large stone statue of a deformed dog. The other side of the arch is empty, making for a curiously unbalanced structure.
The school was a front for a Gozerian cult. Statements which the poor students made to investigating officers, as recorded in county court records, indicate that Bent trained each of his charges in the sinister practice of Gozer worship.
Finally in 1878, Bent was ready. He assembled the students in the building with the oddly shaped roof, and he began the ritual of summoning.
Until this moment in history, it was thought since the ritual was begun, destruction must surely follow. The courage of a schoolboy and the stupidity of the minion Vinz Clortho would prove that long-held belief wrong.
Bent's arcane ritual woke Vinz Clortho first. The boys' accounts became confused here; events are difficult to reconstruct. Vinz Clortho must have galloped past an old plow-horse that the school kept on the grounds, and rather than possess a human form, the demon Keymaster took the horse's body. It was the possessed Clydesdale then, that burst through the doors of the assembly room and demanded in a voice hoarse from centuries of silence, "Where is the Gatekeeper?"
It searched the room with fiery eyes. No on spoke out until young Alan Childress, a sixth-former said to have done the best impression of the schoolmaster, hit on an idea. In a near-perfect imitation of Hawthorne Bent, young Childress answered, "Here I am-I am the Gatekeeper."
The demon-horse spun its head around, perhaps looking for the source of that adult-sounding voice. "Are you the Gatekeeper?" it rasped mechanically at the stunned Bent.
"No, no," Bent stammered. "Zuul is. If you wait patiently, please, we will summon him."
But Vinz Clortho remained fixated on the voice he had heard. "You are the Gatekeeper! I am the Keymaster!" the horse bugled as it leapt towards Bent. The schoolmaster ran screaming.
Neither Bent nor the horse was ever seen again, but several Border farmers reported seeing the running figure of a man pursued by a horse with strange, glowing eyes. Bent never completed the ritual, Vinz Clortho never met with Zuul, and because of the ingenuity and courage of young Mr. Childress, Gozer the Gozerian was not allowed to walk the Earth.
The world at large encountered the Gozerian cult in Vienna in 1899. There, a public outcry arose over a lecture delivered by Dr. Michael Zhorchev, a Serbian surgeon from Zagreb. Dr. Zhorchev believed that human beings were by nature an angry, abusive, violent race that surgery could transform into docility. Zhorchev was laughed out of Vienna, and it was discovered that he actually performed operations on all but one student at a Serbian university, he was hunted down and hanged.
During the hunt for Zhorchev, investigators where struck by the unusual shape of his Zagreb house. Upon inquiry, the searchers discovered that Zhorchev had built the building himself, at considerable expense, after a research trip to Damascus. The roof of Zhorchev's house was precisely the shape of certain odd and ancient ziggurats in the Middle East.
The exact motives of the sinister Serb are unclear, but it is known that he had at least one disciple. Ivo Shandor, an Albanian medical student and research assistant to Zhorchev, fled to the United States and took with him the secrets of the Gozerian cult.
John Tobin went undercover in the Cult of Gozer to research their methods and history for his Tobin's Spirit Guide book. He later published a more in-depth look at the cult in a smaller book titled "The Gozerians Among Us". 
In the 1920s up through 1938 or so, the Sedgewick Hotel played host to the Cult of Gozer.  Evelyn Lewis, a member, often hosted meetings of the cult in her suite at the Sedgewick in the early 1920s.  Supposedly, one of their incentives to draw in Gozer was to summon a hungry spirit that would wreck havoc on the American food supply which would have made them prime targets during World War II.  However, a semi-corporeal manifestation of gluttony was conjured instead. The green ghost stuffed their sacrifice, a live chicken, into its mouth. Luckily, the ghost was too lazy to roam around and eat everything in sight. It remained anchored to the Sedgewick for decades. The Cult of Gozer's background was included in a revised edition of Funder's Cults of the Northeast.
At least one faction splintered from the Cult of Gozer and its remnants formed the Temple of the Divine Father, dedicated to worship of Gozer's father, Koza'Rai.  The Temple was active in 1953 and facilitated the birth of Koza'Rai's half demon child Rachel Unglighter.
The Cult once consisted of many prominent people of wealth and power. However, the Cult reached its true form when all of them died, re-emerging as a ghosts. The majority of the immortal cultists' powers were boosted by the Black Slime, which is the source of most of their power. However, some of them are enough powerful on their own.
- Ivo Shandor - The legendary architect, inventor, paranormal genius, and homicidal madman. He is the supreme leader and the founder of the Cult.
- Edmund Hoover - Also known as Azetlor, this high-ranking Cult member become a keeper of the library's Mandala. He was the one who discovered the secrets of Gozerian lore from the library, and now rules a twisted ghostly version of it.
- Cornelius Wellesly - A high-ranking Cult member who is the keeper of the museum's Mandala. Once was a powerful tycoon.
- The Spider Witch - True name unknown. She was a serial killer who acted under Ivo Shandor's commands, and a high-ranking member of the Cult. Now she is a keeper of the hotel's Mandala, and a powerful, spider-like ghost.
- Cultist Summoners - The ghosts of powerful high class cultist members who possess summoning magical abilities. Minions of Ivo Shandor.
- Cultists - The ghosts of powerful cultist members. Powerful beings on their own, but still a minions of Ivo Shandor.
- Black Slime Monsters - The most powerful of Black Slime-dependent ghostly cultists. Once a wealthy, powerful people.
- Black Slime Ghosts - Cultists which existence is completely binded to Black Slime.
- Black Slime Fiends - The lower-ranking cultists re-emerged with the help of Black Slime, becoming a mindless minions.
- New York City Public Library - Mandala location, holy ground and a source of knowledges for the Cult.
- Natural History Museum - Mandala location and holy ground.
- Sedgewick Hotel - Mandala location, holy ground and presumably a place of sacrifices.
- Shandor Island - Mandala location, holy ground and a Cult's hidden lair. The source of Slime and the place where the captured Sloar is kept.
- 550 Central Park West - The building which serves as the portal through which Gozer will enter the world. The four Mandalas were to power it, supplying the god with enough energy to gradually manifest in full power.
- Central Park's Cult Cemetery - The Cult's post-life hidden lair and a new target for Mandala energy.
- Black Slime - The main source of Cultists' power. It originally appears everywhere the ghost world collides with ours. The cultists were able to mass-produce it using the captured Sloar. Many of the Cultists's ghostly reincarnations are dependent on this slime.
- Gozerian Codex - The mystical tome that is the spellbook belonged to the Gozer Worshippers.
- Mood Slime - A special variant of ectoplasm created by Ivo Shandor from the Black Slime. Has an ability to open rifts to ghost world. Also could bring objects to life and be infused by both positive and negative energy. Comes in different colors.
- Mandala - A powerful magic that focuses the spirit energy and empowers Gozer's destructor form. Later used to empower Ivo Shandor himself. There were total four Mandalas in New York.
- Sloar - Being from another plane of existence, this massive creature was captured by the Cult and used to mass-produce the Black Slime. Also served as the Mandala guardian.
- Temple of Gozer - The sacred Shrine of the Cult's god, Gozer.
- On page 21 of Ghostbusters Issue #6, part of the seal can be seen in the top right corner.
- On page 19 of Ghostbusters Volume 2 Issue #12, the Cult of Gozer seal appears behind Ron Alexander.
- On page eight of Ghostbusters Volume 2 Issue #15, Kylie Griffin alludes to the Cult of Gozer. 
- On Ivo Shandor's Ghostbusters: The Board Game Character Cards, the Cult is mentioned. 
- In Ghostbusters Annual 2017, on page 41, the cultists who appear are Spider Witch (below left of Chairman), The Chairman, Ivo Shandor (green on robe), and Edmund Hoover (right of Shandor).
- Chapter 22: Holding Cell
- Alluded to by Egon Spengler and Peter Venkman.
- Chapter 22: Holding Cell
- Ghostbusters: The Video Game
- IDW Comics
- Insight Editions
- World of Gozer Audio Displays; Board of Trustees area (2009). Ghostbusters: The Video Game (Realistic Versions) - Museum of (Super)Natural History Level (2009) (Video Game). Terminal Reality. Audio Display says: "The World of Gozer is one shrouded in mystery and lore even to this modern day. The earliest known appearance of the secretive deity is in pre-Sumerian culture around 6000 B.C."
- World of Gozer Audio Displays; Board of Trustees area (2009). Ghostbusters: The Video Game (Realistic Versions) - Museum of (Super)Natural History Level (2009) (Video Game). Terminal Reality. Audio Display says: "Few historical records of this period exist and much of what we know about the time period relies on conjecture, legend, and unverified occultist text. Gozer was worshiped by a large Sumerian subculture dedicated to destruction and chaos. The Gozer Cult waged a long protracted war with the followers of Tiamat in the 4th millennium B.C."
- World of Gozer Audio Displays; Board of Trustees area (2009). Ghostbusters: The Video Game (Realistic Versions) - Museum of (Super)Natural History Level (2009) (Video Game). Terminal Reality. Audio Display says: "The armies of Tiamat defeated the Gozer Cult and the resulting mythology tells of the victorious Tiamat banishing Gozer from this world."
- Grab from DS version 'Collect the Collection'
- Cultist Tobin Scan Notes
- Egon Spengler (1984). Ghostbusters(1999) (DVD ts. 1:12:03-1:12:12, 1:12:19-1:12:36). Columbia TriStar Home Video. Egon says: "The architect's name was Ivo Shandor. I found it in Tobin's Spirit Guide. He was also a doctor. Performed a lot of unnecessary surgery. And then in 1920 he founded a secret society...After the First World War, Shandor decided that society was too sick to survive. And he wasn't alone. he had close to a thousand followers when he died. They conducted rituals up on the roof, bizarre rituals intended to bring about the end of the world, and now it looks like it may actually happen!"'
- 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #5" (2017) (Comic p.23). 101 Class Notes reads: "(In fact, Tobin actually went "undercover" in the Gozerian cult to research their methods and history, and supposedly went more in depth on this in a separate, smaller, book titled The Gozerians Among Us. The print run was marginal, and no copies are believed to exist.)."
- Ray Stantz (2017). IDW Comics- "Ghostbusters Annual 2017" (2017) (Comic p.41). Ray Stantz says: "It says the hotel played host to our friends in the Gozerian Cult up through 1938 or so, where as an incentive to draw Gozer to this dimension... they conjured a semi-corporeal manifestation of gluttony."
- Narrator (2016). Insight Editions- "Tobin's Spirit Guide" (2016) (Book p.14). Paragraph reads: "A hotel resident in the 1920s, Evelyn Lewis, was a member of Ivo Shandor's Gozerian Cult (see section V for more information), which often hosted meetings in her suite."
- Ray Stantz (2017). IDW Comics- "Ghostbusters Annual 2017" (2017) (Comic p.42). Ray Stantz says: "The author supposed they were trying to conjure a true hungry spirit, a ghost whose appetite can be never be sated. Tobin agreed with that interpretation."
- Entity Datasheet (2012). IDW Comics- "Ghostbusters Volume One Issue #6" (2012) (Comic p.22). Datasheet says: "Unaware that the temple was in fact the remnants of a cult that had splintered from the now famous Gozerian cult headed by Ivo Shandor, Unglighter gave herself completely to Aston and the church."
- Kylie Griffin (2014). IDW Comics- "Ghostbusters Volume 2 Issue #15" (2014) (Comic p.8). Kylie says: "Even their followers have been at odds since forever."
- Ivo's Character Card (2015). Cryptozoic Entertainment- "Ghostbusters: The Board Game" (2015) (Character Card). Card reads: "As a human, Ivo Shandor was leader of the Cult of Gozer and the architect who designed the building at 55 Central Park West as a giant altar to the Sumerian God of Destruction."
- 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #5" (2017) (Comic p.23). 101 Class Notes reads: "The information contained in this Spirit Guide was extensively used to research both Gozer, its cult, and its agent on Earth, Ivo Shandor."
- 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #5" (2017) (Comic p.24). 101 Class Notes reads: "A set of four books that springboard off the research begun by John Tobin on the Gozerian cult, and takes a closer look at interdimensional theory."
- Narrator (2016). Insight Editions- "Tobin's Spirit Guide" (2016) (Book p.12). Paragraph reads: "Our best guess at the ghost's origins dates back to the apocalyptic Cult of Gozer, which held meetings at the Sedgewick in the early 1920s, went awry."
- Narrator (2016). Insight Editions- "Tobin's Spirit Guide" (2016) (Book p.81). Paragraph reads: "Gozer's followers were constantly trying to compel their god into triggering Armageddon."
- Narrator (2016). Insight Editions- "Tobin's Spirit Guide" (2016) (Book p.87). Paragraph reads: "As noted, Shandor was a lover of mythology, and his readings eventually led him to become fascinated with the beliefs of the Cult of Gozer."
- Narrator (2016). Insight Editions- "Tobin's Spirit Guide" (2016) (Book p.88). Paragraph reads: "According to the writings of the Gozerian Cult, circa 2800 BCE, it was Tiamat's followers---under her direct guidance---that were responsible for Gozer's banishment from this plane of existence."