It appears that the general accepted procedure in most "singaround" song sessions that I've been to is:

You don't sing along on the verses unless expressly asked to, but if the song has a repeating chorus, you assume that it's accepted and normal to sing along on the chorus unless expressly asked not to, but not so loudly that you take over from the singer. (That's what "chorus" means!)

If the singer starts singing unaccompanied, you don't play your instrument unless the singer has asked for accompaniment. If the singer starts out playing, it's acceptable to play along unless the singer asks that others not play. In either case, anyone playing along should be very careful with his/her volume, not to overpower the singer. Especially b*nj* players and fiddlers.

Are the accepted rules different elsewhere?

P.