This question is about the same as asking "Are people elitist?"

Without question, some are. Some people whether they have anything to strut about or not, don't want to play with weaker or inexperienced players. Sometimes with good reason. I'm visiting a friend out of town this week and we went to a session last night where I'd played on a previous visit but hadn't played with any of the people there. One person visibly winced when I asked (and I always ask at a new session) if I could sit down (a bodhran, oh dear), but nodded me in. Once I played without being disruptive, they were ok with me. Then some new bodhran-owner (not my term)who's been casing this session, used me as his "in" without asking any of the regulars, as if I were in any place to invite anyone else, and proceeded to demonstrate why people are sometimes skeptical of letting new people into a session.

IMHO a session should be reasonably inclusive but one should meet some standard of competency to be included. How high the bar is at a given session varies but a general working knowledge of one's instrument(s) is not too much to ask.

Rich