I might have differed on the very fine singer in those circs, and have found the joining in acceptable (although man or woman should "know their limitations") - but in a singaround or song-session or session I thinnk it is for those who do not want others to join in to make that very clear, and to consider whether they are the ones being conceited or arrogant. If doing a song or tune in an unusual way in a singaround or session I think it is also good manners for the "leader" to make it clear in advance that would-be participants need to listen and follow.