I remember hearing a live concert by folk singer Don McLean, who got an entire audience to participate in a round called "By the Waters of Babylon". The first run-through was a little rough, but he started over again and it was beautiful. Once all of the different sections got to the middle of the song, the harmonies were just amazing.
One of these days I'm going to convince my church's choir director to find an appropriate round and let us sing it for the congregation. I just think the sound of different melody lines harmonizing with each other is breathtaking when it's done well.