We're on the same page Dave. I have a cheap cordless mic that I let the guests use. At least twice, the battery has mysteriously gone out right in the middle or between songs. (Maybe my hitting the mute button had something to do with it too.)
Eddie, I always ask the celebrities if they want to sing, but I do it privately. If they are aggreeable, then I ask them over the mic.
Incidentally, a real pro will NEVER ask to sit in. Even if he is so inclined he will wait to be invited.
DonM