I would not let anyone who's not a member of my band perform during a gig of mine, unless it's been arranged beforehand and I know they know what they're doing and we know the song/tune.

After all, it's my gig, and the audience (presumably) is there to see my band, not some random stranger. Too much possibility of embarrassment... and if I'm gonna be embarrassed in public, I'd want it to be for something I did. :-)

Cheers,
Michael