Rob, you’re asking questions that for once, I’ve never actually heard asked ever!
There isn’t a technical answer, and the terminology in this case is local. Certainly not international, and it’s going to be one-off those no correct answer. I’ve heard ‘subscriber’ used too. As for naming, well it depends on who they are? If a talkgroup (my preferred terminology) is in my case the guys who work on stage, then their group would be stage, which is what they would be called on paperwork, other than LX, and FOH who also share a channel. I guess I’d pick ‘members’ if I needed a description of people in each one. I’d also say something like “which talkgroup are you in?” But ‘on’ would do too.
Users, members or subscribers depends on who they are, off the radio system which is just the tool. Members of a club, subscribers on a bar security system and users for when presented with a radio by somebody else? There seems no need for rigidity or conformity?