I've always called these "sit ins" as well.
I don't really like when people ask. Sometimes it's good, sometimes horrilbe (either on their part or my part)
The truth is, I can not say "no." So any tips on how to just say "no" to folks that want to sit in? I would feel like a mean jerk saying "no"
"No, I'm sorry." (Simple as that.)
or
"No, I'm sorry, my insurance policy doesn't provide coverage for anyone but me and I learned my lesson the hard way. Sorry."