IME most musicians and singers know instinctively when to join in and when to listen (e.g. for a slow air on a flute, or an unaccompanied "serious" song). But I have had people at singarounds attempt to accompany me when I've been doing a song that I think would have been better left to me, especially as on the particular occasion I'm thinking of, the guy couldn't play the right chords anyway. However, it's just a singaround and not a concert at the Albert Hall, so I don't let it bother me. Usually people will invite you to join in by saying something like "It's in A, if anyone feels like playing along", to make it clear.