From my understanding and how it work in Britain the show ask who they want. They sometimes may be offered certain people but in the end the asking is the show and down to the individual if they appear unless their party has told them to do so if asked.
Hi snugs ... hope you're having a nice Sunday, and that your Mum and Dad are doing much better. Guess what's falling outside my window again? PS ... MORE white stuff ;) .. ooh, this is getting to be a challenging winter for me.