I will let anyone, at any gig (except festival shows where I don't feel that the option is mine to grant)come up and sing or play - but they will have their own instrument or be accompanied by me and my guitar. Simple; plain; and, across-the-board. I cannot accurately gauge a strangers emotional and/or physical state in a pub situation -and I shouldn't have to - therefore, no one uses my guitars, period. God, I'm stingy; but my guitars all work just fine. Cheers.