Whether you are unaccompanied or not you have the right to specify whether you want anyone else to join in. But it does depend very much on what you are singing, and whether you know the would be accompanist. Some songs are best unaccompanied, some unaccompanied singers appreciate having some instrumental backing because it's not something they can provide for themselves, some players are good at it, others..... But make your preference known before you start. If you want others to join in tell them what key you're in, possible not easy for an unaccompanied singer. If no-one invites you to sing invite yourself, via whoever seems to be running the session. Don't sit there doing nothing but fuming, or you'll end up going home thinking 'I'm not going there again'.