I've been to a fair number with some friends who are professionals. These are always in someone's house, not in a venue. We go around once and everyone performs solo, without anyone joining in. After everyone has finished the jam begins. They do it that way so the amateurs (like me) won't feel too intimidated to perform. No solo, no stay!
Once the jam begins everyone is welcome to suggest songs and join in on other's songs.
Jack Murray