From Muppet Wiki

The Vile Bunch refers to the group of Muppets who appear with Alice Cooper during his stay on The Muppet Show in episode 307. Working for an unseen employer set on gathering souls, Alice has brought his own monsters -- a snake, Beakie, Purple Heap and a monster backup band.

The band -- including Chopped Liver, the Flower-Eating Monster, Silver Beak, a drummer, and a guitarist -- performed with Alice on his hit songs, "Welcome to My Nightmare" and "School's Out."