She was helping with an RP back on oldguild that had an antagonist group kind of like that actually. Was a radical faunus/faunus sympathizer group that was kinda driving grimm to attack cities and whatnot somehow. Was a while ago and dont really remember details but she could get behind the idea. Cult could be a pretty interesting spin on things, worshiping grimm or somthing. 

Maybe go with kind of a twin organization type thing. Could see something like a cult operating out of forever fall(or somewhere secluded) with ties to gangs or something in the city to do the dirty work. Cult could be trying to study the grimm and whatever other weird cultly activities they could get to.