Quite a few YouTubers have been jumping onto the SEA lore. I'd suggest Offhand Disney's series, mostly because he basically just read the articles I wrote on the subject for Disney Wiki instead of bothering to actually come up with a script.
The basic gist is that SEA was founded in 1538 during the Age of Exploration, with the Fortress Explorations attraction at DisneySea being their original headquarters. Most other attractions tied into the group are from a late 1800s generation.
Some of these members include Jason Chandler (from the unbuilt Discovery Bay and early Big Thunder backstories), Barnabas T. Bullion (owner of the Big Thunder Mining Company), Harrison Hightower (Tokyo's Tower of Terror), Lord Henry Mystic (Mystic Manor), Dr. Albert Falls (founder of the Jungle Navigation Company and the Jungle Cruise), Captain Mary Oceaneer (treasure hunter found on the Disney Cruise Line and at Typhoon Lagoon), and Merriweather Adam Pleasure (from the long-gone Pleasure Island with the Adventurers Club being retconned as his chapter).
There's also links to the Indiana Jones franchise, 20,000 Leagues Under the Sea, Island at the Top of the World, and Atlantis the Lost Empire. Also, Harrison Hightower's brother George was the last victim of the Black Widow Bride in the Haunted Mansion.