I’ve gone to three meetings of two different ones in my hometown over the past year, just because I noticed they were reading novels I’d recently read, and two had the authors, who had a local connection, dropping in at the beginning.
All three meetings were quite weird. The one without the author present featured a lot of people complaining because the novel wasn’t a completely different kind of novel eg they’d have preferred it to be a suspense novel when it was a coming of age novel. When I said ‘But it’s not trying to be the kind of novel you’re complaining it isn’t’, they got very cross.
One that had the author there at the start was all nice to her face and started ripping the book apart the moment she left.
The other that had the author present consisted entirely of very young fangirls (the same author also writes YA) saying ‘I loved this bit!’ and one who rolled up her sleeve to show she had a line from the novel tattooed on her arm.
I conclude that book groups, at least in my neck of the woods, attract quite odd people.