Wouldn't it be an idea for the person who's running the singaround to 'set out' the way they are running it and then no-one has any excuse to be 'jumping in'. All regular singaround members know the score, even if they don't always agree with the way a person/people runs them.
Especially galling for the people who've been waiting to be asked to sing, and who probably got there early - grrrr