That was their closing number, but if you listen, only the Muppets are singing it.
I revised the non-singing guest list:
Season One: Juliet Prowse, Harvey Korman, Peter Ustinov, Phyllis Diller, Mummenschanz
Season Two: Don Knotts, Edgar Bergen (the Muppets sang "Consider Yourself and Charlie just made wisecracks during), John Cleese
Season Three: Spike Mililgan (arguably- he doesn't sing "It's a Small World" but toward the end, he hums it... before spitting out a ping-pong ball)
Season Four: Shields & Yarnell, Victor Borge, Jonathan Winters, Doug Henning
Season Five: Jean-Pierre Rampal, Wally Boag (he speaks some lyrics in the closing # but never sings them)
For the longest time I thought Brooke Shields didn't sing on the show, but she does actually sing the first two "I'm falling"s.
David "Gorgon Heap" Ebersole