Well, Nigel I beleive usually left when ever he felt like it. Or just...didn't care. Trumpet Lady only left ONCE. ONCE! She lives in the pit.
Rowlf has to leave the pit alot, and usually, get someone to lug that piano on stage for him, like Bo. But when he's not playing it, he's usually in Vets Hospital, which is usually added to with a keyboardish sounding organ, I supose Dr.Teeth plays the intro for that.
Floyd always leaves the pit. I still think it's because he finds the theme so square, he'll only do it twice. But usually, he's seen in the pit for the guest stars that show the pit. Maybe out of respect for his fellow band mates?
Zoot just wanders about, wondering where he is.
Animal, well, lets face it, would you tell him to stay in that over packed Pit for long?
Lips usually leaves the pit to join the Mayhem Band, plus, the tension between him and Trumpet Lady could have been cut with a butter knife.
Janice joined in the pit a few times, usually either next to Floyd, Zoot, or Lips. She just joins in if they need an extra guitarist it seems.
Crazy Harry, seriously, would you stop him from leaving?
My question is, how come Dr.Teeth was only seen in the pit for a Muppet Christmas? Its obvious a keyboard is used alot for musical numbers, but he's nowhere to be seen.