Some songs invite singing on the chorus and most audiences seem to know where it would be welcome and where it would be intrusive. A song where the performers have worked out an intricate three or four part harmony would not be appropriate songs to have a sing-along chorus. When I am performing solo, I would welcome audience participation on the chorus by people who knew the song, especially if they can add a harmony. Sometimes the audience members might know the chorus by the time it comes around the second time and sometimes I will outline it for them before the song.