I struggle with stuff like that. I don't like it.
I'm not even that big on folks sitting in. Usually it's initiated by the band leader. The rest of the band members are a lot more generous about sit ins than me. I'm a lot better with it than I use to be. It's really not a big deal. Most of the time it's local guys that are good. However, I think there are some gigs where it's not appropriate
Daryl