Sesamestreet.org has a few clips from an episode with Elmo, Little Chrissy, and Big Jeffy (you can do searches for those characters, though sesamestreet.org spells it as "Jeffie", I can't remember but I think the site also spells it "Chrissie"). I've also seen a Muppet Wiki guide for a different episode which also features Elmo, Chrissy, and Jeffy.
It's sort of confusing. They need Elmo to fill in for Little Jerry, but this is the only time I know of where Little Chrissy (from Little Chrissy and the Alphabeats) was part of Little Jerry and the Monotones, which had a different member names Chrissy. And so the only member of The Monotones we see is Big Jeffy. Somehow I wonder if they just used the wrong Chrissy puppet (though it seems like an odd mistake, as Chrissy and Rockin' Richard aren't referred to by name often). And they wanted Elmo to fill in for the leader? Why not have an experienced member fill in, and have Elmo be a back-up singer?
And there's a funny running gag in that episode where Mike clearly wants to be involved, but the others don't seem to notice or think he'd be a good member. Whenever it looks like they'll make Mike part of it, he's excited, but it turns out what they need him to do is more humble stuff like getting their music and announcing.