I've run a couple of singarounds at Four Fools festival in the past and I get the people who come in when the thing is in full swing - they don't come in at the start when it's empty and no-one can hear their song - see who's singing and make a bee line for the seat nearest to them so they can have a go next and go away (I'd have said "bugger off" but I'm too polite for that). What I tend to do is tell them that I'm going to come back to them after I've been round the people who have been there waiting patiently for half an hour or so for their turn so they at least have the enforced decency to listen to other songs. If they don't want to wait and just get up and go then that's fine by me.

I've also been in the situation where I've arrived at a room with a singaround a few minutes before the advertised start with maybe half a dozen other people, had to wait half an hour before the organiser thought that the room had to be half full before he/she would start, been sat in the wrong place in the room because said organiser decided to start on the left and go round by which time more people have come in and filled the room up and by the time it's got round to my turn I've had to go because I was due on elsewhere.

I can't say I've had "guests" come in demanding spots but I have had "guests" sit there listening for quite a while and stay there well after they've had a go themselves and commented on how much they enjoyed listening to other people.

Cheers
Mark