Whilst agreeing with Joanne about etiquette being stated upfront, how can this be done at a session where people are coming and going at different times? A large unmissable notice perhaps?

Singarounds, just about everybody knows the etiquette, join in if invited, don't join in if not. Newcomers should soon pick up the situation and if not the organiser can easily slip in a general reminder when needed.

A 'session' is a session is a session, a free-for-all. Again, if somebody wishes to sing without accompaniment they should be prepared to state this before starting. Or perhaps bring a pitchpipe in a godforsaken key between two notes so nobody can pitch in with you.