I go to a regular "open jam" kind of thing at a local bar, and while it's truly open, you will find yourself banned from the bar if you don't observe at least some basic courtesy. There's always a mix of musical styles, and anyone who chooses is free to join in or sit out, so it's fairly loose and informal.

Problems are rare, but one woman was recently re-banned simply for joining in. The fact was, she was terrible, plus she was absolutely convinced of her own talent, and no one wanted to play with her.

I always find myself outclassed, because there are some excellent regulars or semi-regulars. I love being outclassed, because it's an opportunity to watch someone who's spent enough time with their music to treat it like and old and close friend. I can watch people and get new ideas, see how they approach their playing and how it differs from my approach, and simply enjoy the great stuff they come up with and do my best to keep up. It's a worthwhile experience, but you can't forget that in any social setting, there are unspoken rules of conduct.