Seems like a fundamental problem is ground rules. I'd never intrude on a paid performer or "open-miker" unless harmonies or accompaniment was requested. Song circles need to be clear as to whether they are going to be sequential individual performances with each one getting to invite (or not) others to join or a rolling group sing with people sequentially choosing songs. In the latter case, the person choosing the song should generally get to set the tempo and tune. My own hate is getting my choice (I do sort of play guitar) and having others sing roughshod over my version.