The primary thing is, FOLLOW THE SINGER. Especially important if someone is singing a different version from the one usually sung at a particular session.
The Friday Press Room session (which Curmudgeon -- Tom Hall -- started over 33 years ago) has been both vocal and instrumental right from its inception. I have always tried to sing songs that either have a good chorus to join in on or a good instrumental back up and break to keep the tunifiers happy. Where I have the most problem (both when I was just a regular and now that I'm leading the session) is when instrumentalists attempt to accompany an a capella song. That can REALLY throw a singer off as English (in particular) a capella songs can't really be forced into a time measure straight jacket.
I've been known to throw things at the offender. (When I'm the singer. If I'm not the singer, I try to get the offender's attention somehow -- yeah, usually by throwing something.)
I DO have the gavel in the car, but I seldom have it at the table when I need it. Sigh. Guess I COULD fix that...