From Disney's Hollywood Studios Wiki
The 501st Legion is a charity group of Star Wars fans that own film quality costumes of clone trooper, stromtrooper, or any other Imperial character. The group has outposts all over the world, and usually appear at special Star Wars events, and other charity events.
 Studios Reference
The Florida Garrison of the 501st appears in the Star Wars Weekends Motorcade every year, being just about every Imperial character featured. They also serve as Streetmosphere in their trooper costumes during the event, while not preparing for the parade.
 Star Wars Canocity
The charity group is named after the 501st Legion of the Star Wars universe. This legion of troopers is assigned to Anakin Skywalker during the Clone Wars and continues to serve him after his transformation to Darth Vader in the Galactic Empire.