The idea of joining in with singers, in particular, and especially if they choose to sing unaccompanied, is definitely not my cup of tea, and unless stated as the norm, or invited, is just plain rude, IMO. However, someone locally has started a few song and music sessions recently which are intended to be, as he puts it, "free-for-alls", and this, according to him, has got some people playing who have never done so in public before. I haven't been in a position to attend any of these sessions yet, and I'm not sure that I want to, but maybe he's right, and that sort of freedom has its place in encouraging some people to start performing. At what sort of standard is another question!

Barbara