Well said Peter. Even in a large audience I can feel embarrased if asked to join in. If an artist isn't really good at this they should avoid it completely. Occasionally a performer is so engaging that you can't help but participate. That's a talent very few performers possess and even for that few I bet the experience is rare for many of them.

I say this and I go out on purpose once a week to sing! Imagine the poor audience member who is reluctant to sing in the shower.

If the performer has been hired to get people to participate (for example with children) then it's a different matter. In a concert or club setting encourage folks if they start spontaneously but other than that forget it.