almost any musician is welcome to join a group...unless their 'shtick' is "being the show".

I think that's the problem he's talking about. He is looking for informal sessions with give-and-take, some band has occupied the centre of the room and is just doing the band thing. I've seen it a lot. Even if they graciously permit you to strum along with them, they aren't going to let any interlopers change the set, the tunes, the songs, even the order in which they are plaayed. And they usually have a claque with them, which is what they are all about really.

We need to start the Campaign for Real Sessions!