I'm almost compulsive about singing along with people -- but I do try to watch and see if it's okay - and in new venues either don't indulge myself or try to keep it very quiet. (it really is hard sometimes to not join in.)

But one of the nicest feelings is to be *mouthing* the words - and have the performer look at you, smile and nod encouragement to join in. Even better is when they come up to you after a performance and gently chide you for not joining in.