Like the Birdketeers, the Grouchketeers are a group of real kids dressed in costumes based on their leader, in this case Oscar the Grouch. Oscar depends on them to help him demonstrate things to the viewer (such as a game of Three of These Things), help him make things trashy or noisy, and sometimes even find him some more trash. Telly was a long-time Grouchketeer.
One early appearance in Episode 1952 featured Grundgetta leading a Grouchketeer meeting and playing "Cross it Out." Grundgetta calls the kids up and asks them to cross out a different thing on her card. David calls her out on something; the kids are good at crossing things out, but she's done nothing. She gets the card and crosses the whole thing out.