I'm no expert but I think the fairest ways is "by time of arrival" which of course will need an aware and organised MC. A singer who turns up at eight should be able to sing more songs than someone who strolls in at a quarter to ten.

I've yet to go to a singaround where everybody couldn't have had a second song though I have been to one where early arrivals got one and later arrivals got four - you just don't go again.

I like to know a bit about the songs people sing though I draw the line at ten minute ramble on how the singer spent his/her weekend and what their journey to work was like this morning. Again it require a strong MC.