It seems like The Christmas Party Sing-Along was just sung by Rowlf (with background vocals by chickens and penguins). I feel like that song should have featured more characters joinning Rowlf (though it's good that he basically has a solo song on the album). It would have been great if Rowlf was joined by Kermit, Miss Piggy, Fozzie, Gonzo, Rizzo, Pepe, Bunsen, Beaker, Beauregard, Bean Bunny, Sam the Eagle, Lew Zeland, and maybe also Johnny Fiama, Link Hogthrob, Dr. Strangepork, and Robin.
I just realised that three of the credited performers who we don't know why they were credited are known to have been members of Muppet Central. Mike Quinn, Karen Prell, and Jerry Nelson have all posted here before.