The interesting thing about sing/play arounds is that the person(s) that are shy and reluctant, but eventually are persuaded to perform,usually put on the best performance.
This idea is perfect for Festivals as most of the performers are unknown to the organiser ,unlike normal pub sessions and it only takes a few guest artists just popping in for a quick play and then going off to their main gig to make it a great evening. It does need an organiser however someone who can find out who the players are.Let them know when their turn is and slot in the guests when they arrive. Cut out the long tune introductions,where the tune was collected etc ,just move it along quickly. It is not a Folk Club it is a session.
The third idea which I forgot to mention is the guest/theme evening featuring a guest who runs a session from the stage on tune collections.
Initially running through the tune slowly for everyone to learn,some issuing dots for those who read music.In this way the whole audience participates in the evening and from what I saw at Cecil Sharp House recently, where the room was packed out for a fiddle tune workshop. A rare occurance at Cecil's.
Al