Such an interesting thread! As a performer I rarely sing unaccompanied and have only a few songs that I sing unaccompanied : 'When a man's in love', and a Hungarian song come to mind. But I run singing classes that are run unaccompanied by instruments. We have so much fun because of it: the group improvises harmonies, vocal accompaniments etc. etc. At times someone accompanies certain songs, but it seems to make the whole group a lot lazier, and very comfortable just singing the melodylines. So for group singing I prefer not to work with accompaniment. (we sing many different styles, it doesn't matter) Listening to an unaccompanied singer can be nice but it not always is... As for instrumentalists (or other singers!) who join in uninvited, that really makes me nervous, even more as a listener than as a performer. Unless the whole evening is set up with the understanding that everybody can join in at any time, I feel that performers will let you know if they want you to join in. If they don't, then there might be a reason (as mentioned before there might be accelerations etc. etc. going on, or the song may simply want to be heard by itself.) As for performances with instruments, especially when there is more than one person playing, I find that the sheer volume of the speakers sets people into talking. Try turning your volume down and you will draw people in. Turn the volume up and people will shout in eachothers ear instead.
An