Grips
  The grips were the kids who did the set changes. They dressed in black so that when they were changing sets and the lights were out people wouldn't see them. If they dressed in white they would be more visible. The reason they could not be  visible to the public is so that the public wouldn't pay attention to them when the performers were on.
"My favorite part was getting the sets on the wheels." - Andrew
"My favorite part was moving the sets back and forth." - Jackson 
"My favorite part was moving the sets." - Rachel