Try to blend in. Wear black. Look shifty. Record right from their midst.
That sounds on the face of it, like pretty good advice, but I think these fiends and vampires could “make” you pretty quickly. Undercover cops in my experience at 60s rallies always stuck out like a sore thumb, but one would really have to practice their acting before going out. ALSO, they ALL may know beforehand who is in their ranks already and who they expect to “join” on the spur of the moment, if anyone.