if singing a 3 part round, for instance, it works best to have 3 strong singers, one in each group, who know the song and can sing their part to lead the others in the section. Perhaps it would work best if they face their group so the others can hear and follow. of course you would need a group of people willing to participate. I once sang a round while performing solo and was pleasantly surprised that several audience members took up the parts and sang them out (white coral bells....girl scouting will do that to you!!)