Harmonies and dirges need not be the same thing at all! And slowing down choruses and making them dirgy (is that a word?) is by no means the prerogative of harmony singers! Good harmonies sung by good singers can make choruses live and breathe in a way that straight unison, mostly, doesn't.

As has been said somewhere above, the best answer for anyone joining in with choruses - or refrains - is to watch (and listen to) the lead singer, so that one doesn't go one's own sweet way either in terms of speed or version. It's called respect... and in the end is more satisfying for everyone.

Barbara