Great advice MAB! I personally don't even try to "help". I don't harmonize that well because I never have practiced it. And I don't play any kind of simple guitar to join in either. Like you said, the first responsibility of a stage sharing writer is, "DO NO HARM!"
Brings up a BAD story. At the Frank Brown festival last year, Don put me on stage with another amateur and one of the hit writers that had come down. The other amateur was on one end, I was in the middle, and the pro was on the other end. I told the pro if he didn't mind, I would let the other writer go first, then me, then he could close out the set. I thought that would be respectful? When it came my turn, I started my song, and all of a sudden I was hearing these really terrible notes being played! I was like, "WOW, am I really playing that bad?" Totally messed me up. So, I finished and the pro began. Then I heard those same terrible notes again! Turns out the other amateur was filling in with his own made up licks! The pro and I looked over at him and kind of shook our heads like, "No, just be quiet". He finally got the message, but not until after the damage was done. I apologized to the pro afterwards. Then, I was talking to some other friends and they said, "Yes, he was doing that while you were playing too!" I guess some people just like to show off, even if it makes them look like an idiot!
phil g.