When I sing, I've never been a huge fan of the "crashers". Mostly because I'm announcing stuff like "Make sure all the names of everyone singing a group song are signed up, to help keep the rotation fair, etc...". Kind of looks like I'm playing favors if someone jumps in & helps me on a song, when I won't let anyone else do it for other customers, 99% of the time, if their names weren't signed up for it. Guilty conscience
.
Once in a blue moon it'll happen, and I'll just give them a smile and shake my head "no" before they make it on stage. More often then not it will be someone who is already singing on the list and couldn't sing again at that point in the show anyway without "breaking the fairness rules".
Luckily, I try to sing songs that most people wouldn't wanna join in on anyway.