I think we're talking about a range of settings and situations which vary quite a bit. At one end there's the large, formal concert with a Big Name who will in most cases be amplified, and thus less likely to invite or encourage joining in, and it's less likely to happen anyway. At the other end of the continuum, there are informal singarounds and house parties where a number of people get together to sing. In these circumstances, one person may lead a song, but joining-in is expected and encouraged - it's a very natural form of human social activity, and more or less the vocal equivalent of the open music session in a pub.

In between these extremes there are all sorts of other situations where singing occurs - folk clubs, festivals, singing days or weekends, private parties, etc. It's in these in-between situations that people may not share the same expectations, and misunderstandings may occur. Some events are set up with the expectation that everyone may sing if they wish (you try telling 'em not to join in at the Anchor Middle bar at Sidmouth!), while others may consist of a succession of mini-performances or party-pieces by individuals or small groups, and attentive silence is expected.

I really don't see how either of the extreme positions ("no one has a right to join in a song with another person unless explicitly invited"; or "anyone should feel free to join in with another singer unless asked not to") could be made to apply to all situations. People just have to use sensitivity and common sense, and not get too upset if others don't interpret the "rules" in the same way.

Marje