I agree with Pip, I'd probably simply sit there and wait and wait ...and wait if it were a 'jump in' session. The unaccompanied singer is also at a disadvantage to someone using a guitar (and I think we tend to be in the minority), because the guitarist can just loudly strum a chord or start tuning up and everyone will look and settle down. It's a way of getting your foot in the door so to speak, without formally announcing to the group "I've decided it's my go now!".