Yes,it's fine if the performers have ASKED for it,but otherwise not really.In my experience,most artists will say something like "please feel free to join in on this one if you like", or " I /we could do with some help in the chorus " , or whatever.What I hate is when a bunch of prats get up and start "dancing" in front of the stage and block one's view.-----And why oh why do some people have to yell and shriek and whoop at the end of each number ??It's supposed to be a concert,not a f----king Comanche war-dance!Gives me the shits..