There is nothing I can really add to the topic, I heartly agree with everything that has been said here. I?ve been in that situation where someone unknown comes from the dark and says Hey you guys sound really good, I will play some tunes with you and I simply say: No, pal, this is a rehearsed show -even if it is not- and we cannot allow changes to it. Then there?s the occasional guy that comes and goes I wanna sing this or that tune, give me your guitar, who gets a blunt NO from myself, without any further word. I have seen - and suffered- one too many times all sort of accidents and mishaps in the middle of a performance, that I REALLY prefer to be seen a the ogre-like guy, rather than letting anyone screw my WORK. Of course, with a handful of acts I play with we DO invite other musicians we know, and they join us, not only for songs we perform, but in several cases we do know some of THEIR original tunes and act for a minute or two as their backup band. But that is very different from trying to steal the show, which is what makes me think of this whole scenario. Now, if it was a Jam it would be a completely different situation (although I do think there applies some etiquette as well), but I try my best to NEVER let it happen on a gig. There go my 0,0002 parts of a cent...