"How can you tell a Grouchketeer?" "He's the one with the thumb in his ear!"
The Grouchketeers, like the Birdketeers 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, & sometimes even find him some more trash. Telly Monster was a long-time Grouchketeer.