Rule 1 should be that everybody who was there from the start gets a turn in preference to later arrivals.

But it can be very difficult with singarounds at festivals, especially where the time is strictly limited. I've seen plenty where you don't get once round in 3 hours. A knowledgeable host should be able to spot (and not call) those who've been to another simultaneous SAR, had a turn and then come to yours for a second helping.

I've no sympathy for artists who'll turn up, expect to sing immediately and then leave, but I think it's fair that artists (and stewards and even morris dancers) should be moved up the order a bit if they are booked to be elsewhere soon, so long as they have spent a reasonable time in the SAR waiting their turn with everybody else.