I don't know what people expect from a session. There's a subtle difference between a session and a singaround - the latter being a gathering where people perform in turn, and the former being a general joining-in - and I think that everyone in the company should know which applies. If someone at a singaround invites everyone to join in, then it becomes a session for the moment. If someone at a session really doesn't want others to join in - which may be a perfectly reasonable wish - then I think that should be made clear before the start of the performance.

I don't thrash all the way through a session myself - it's actually good to stop and listen and relax and get a beer at points during the evening - but I make absolutely no objection to anyone joining in with me. If this means - like Tuesday just gone - that I get a snare drummer, a bohran player and a bones player accompanying me very loudly, well that's the name of the game! (It wasn't the greatest sound in the world to be honest, but it's what I expected from the evening).