This is how flyball is describer by the AKC:

Flyball is a unique dog sport in that it is made up of a team of dogs and handlers. There are four dogs and handlers that complete in each heat of a race. Up to two additional dogs can be named to a team to serve as back-ups. The six dogs can even rotate in and out of heats during the course of a tournament – this is the decision of the team captain. In addition to the dogs and handlers, there is one person loading balls into the box — the Boxloader. Teams may have additional people available to help collect loose balls and to help call the passes at the start/finish line.