700 members may be a lot for my group, but who knows, so I'll explain
So in the Chaos Prevention Canon, Kee Ellis makes a name for himself by progressing through ranks of a ~50 member cult and also by helping other members. he eventually branches off into his own group (C.P.A.) with his own level (Level 352) he made through the help of an anonymous being (bot the one the cult worshiped.
The cult disbanded and half the members joined the C.P.A., and the other half either joined the M.E.G. or another group that was also rising in popularity (Crimson Wing), eventually many of the people who had been helped and/or heard of Kee's actions joined the C.P.A. and this brought the group to about ~75 members after a month.
Kee's group had continued to help other survivors along with occasional arguments with M.E.G. for reasons I have yet to fully determine.
About 2 years later, with about ~230 members, the C.P.A. is attacked by M.E.G. (reasons still not determined), and this results in a large scale war lasting about 4 months, also halting the member gain of the C.P.A. (due to possibilities of spies along with member deaths) and making it only ~300 members by the end of the war.
The war ended because there are effects of the war efforts from both sides that affect various levels of the backrooms, (more entities, more hazards, etc.) and the C.P.A. attempts to reason with the M.E.G. to end the war and it works. The M.E.G. goes back to doing whatever they do while Dr. Kee Ellis attempts to fix backrooms levels.
The C.P.A. allies with a mercenary group called Crimson Wing which rises their popularity, bringing it to about 500 members.
The rest of the members join between some point of the alliance and the second M.E.G. war (I have yet to write or determine the lore for)
You're welcome for the lore dump.