I've been to several differnt formats of singaround and session in the last year or so, and at none of them has anyone "jumped in" or tried to hog more time than anyone else.

I've been to sessions where performers well-known to the MC seem to get more time than others, but none where people have shoved themselves forward. I guess that I've attended sessions/ singarounds at 8 different venues in that time and just never seen this behaviour. However, it sounds widespread from some of the comments on here so I guess I've just been going to the wrong (or right!) events.