In principle I would like to say no, but in practise I sometimes say yes. I even let Richard Who Wears Shorts get up and play the ukulele play with us, which seems to happen quite often at gigs as he is an indefatigable follower of music. On one occasion Boy George joined us to sing backing vocals on Iko-Iko, and he didn't even ask permission! That happened to us with Julian Clary too, and we thought the Boat Band might become rather unlikely gay icons, but the community seems to have gone back to Judy Garland.
   As a rule, I would say avoid people who want to sing the Wild Rover or Country Roads.