Hmmm. I guess I would echo some of the others in asking exactly what type of a situation it is. I've been in jam sessions but I also just did an in-the-round thing and none of us played on the others' songs. 'Course we were all playing original songs. Even in a jam situation, though, I won't play unless I know reasonably well what I am doing. I think most people who have any manners will refrain from playing on a song they don't know.

And I say, if this is a gig you play regularly, go right on and keep playing it and don't be defeated by the occasional jerk. :-)