Short answers, pulled from the story.
The 501st Legion was founded in August 1997 in South Carolina by Albin Johnson and Tom Crews, who set up a website called "Detention Block 2551" to share photos of themselves in homemade stormtrooper costumes.
The 501st Legion has over 14,000 active members worldwide, with over 26,000 approved costumes. The organization operates on six continents across more than 60 countries.
The 501st Legion is Lucasfilm's preferred Imperial costuming organization, though it is not formally affiliated with Lucasfilm or the Walt Disney Company. A mutual understanding reached between George Lucas and founder Albin Johnson in January 2007 allows the Legion to use copyrighted Star Wars characters as long as members do not profit personally from their costumes and represent the franchise respectfully.
The 501st Legion first entered mainstream Star Wars canon in 2005, through the novelization of Star Wars: Episode III: Revenge of the Sith, where the blue clone troopers led by Darth Vader into the Jedi Temple are officially designated as the 501st Legion. Author Timothy Zahn had earlier incorporated the unit into his novel Survivor's Quest in 2004 with Lucasfilm's approval.
In 2016, 501st Legion members donated over 182,000 hours of community service and raised over $889,000 in direct donations, while also participating in events that helped raise over $46 million for charities worldwide. In September 2016, the Legion announced a first-of-its-kind international endowment fund with Make-A-Wish, with a goal of $150,000 by 2021 to fund Star Wars-related wishes for children with life-threatening medical conditions.
The Galactic Academy is a joint group formed by the 501st Legion and its sister club, the Rebel Legion, for Star Wars costumers under 18 years of age. Unlike the 501st, it has no minimum age requirement and no strict costume standards, and it covers both hero and villain costumes.