Yeah, well these situations can turn on you too. I'm afraid that I would not have allowed the "guest" to work with me.

I recently was hired to play for a high-end retirement community and was asked if I would mind "if some people got up and performed before I started or in between my sets". I politely told the activities director that I'd rather that they allowed me to do my normal show (all-instrumental, no "guests") to see how they liked my presentation and so I could tell what kind of problems I would have if I allowed "guests" in the future. Good thing - after I set up and before I even started, one guy came up and wanted to play his specially-prepared CD because "he knew what the people wanted to hear". I refused and actually I use an MP3 player for break music and had no CD player anyway. He left... a little while later he came back with a CD player and while I was greeting the people as they came in, he went to the stage and starting uplugging my equipment from the house PA and hooking up his CD player. Security escorted him away (again politely). Meanwhile as I was greeting people I was being told on all sides that I should let this person or that person sing with me "because they were very good". Again, politely I told people that tonight would be all-instrumental (for a change) and there would be no guest singers. After the gig I was told by everyone how nice it was "that I didn't let anyone ruin the show" - apparently the "guest vocalist" routine gets out of hand a lot.

The end of this story is that I was rehired for early next year, with the same request that I allow people to sing with or in-between my sets. When I told the director the problems I had she relented and said she would arrange a "talent show" for the residents instead. I'll be playing solo, and I'll be returning by popular request.