there oughta be something in the thread title saying "for guitar players" *smile*..(although there IS some nice general advice about courtesy)

As an autoharp player, (and sometimes recorder player) and a relatively amateur musican, there are lots of things I can't follow anyway...so I put my ear on the 'harp, strum softly to see if I have the key (whadda ya' MEAN in "B"?").. and am in tune. Then, if it is not too complicated, or if it is obvious that I KNOW the tune the way it's being played, and if other instruments will not drown out the singer, I play along. I do not even pretend to play in a singer starts an unaccompanied ballad, or if a musician is using more than 4 chords, as I'm sure I will miss some of them.

Yep...that means I don't play on everything...and that's just fine.

There rules are especially important if the leader is an average musician who will be bothered and unsure by hearing others doing different stuff than they are doing! (well, there is one local exception who we usually try to overwhelm when he begins murdering a common selection....but...)