This is a common exercise that can be facilitated in many ways. We’ve tried it with up to 50 people, but it can be done with just a few. I don't know where this tradition comes from, but many use it in theatre training. The group lies down in a circle on their backs with their heads facing the center. If possible their shoulders should be touching. The group starts with a simple tone vocally and lets it develop over time. It gets the group listening and responding to one another and opens up good discussions about sound composition.