Best to assume that musicians and other singers want to join in. They've had a few beers, they're there for relaxed fun not a formal concert. That may mean some participants will be boorish, so best to be brief and clear from start about what you're going to do and what kind of participation you'd like. Actively inviting some very specific accompaniment (just 'drones' or 'drums' say) might be preferable to saying nowt and assuming the resident guitar hero won't want to spend the bulk of the song "discreetly" trying to figure out what key you're singing in..