Wong is Doctor Strange's buddy and fellow sorcerer and was Doctor Strange's tutor during his magical arts training.
Lady Sif is a close friend and ally of Thor and the Warriors Three. She's one of Asgard's most dangerous combatants, and she's been on numerous adventures with Thor across the Nine Realms.
In the MCU, The Dora Milaje are the world's most ferocious troops, and Okoye leads them. Her role is to be loyal to Wakanda's monarch, but her actual allegiances are with T'Challa and, subsequently, his sister.
Bucky Barnes, or the Winter Soldier, is a lethal assassin. He and Steve Roger have been great friends since before they served together in World War II.
War Machine is a United States Air Force officer and one of Tony Stark's most trusted allies, James "Rhodey" Rhodes. Tony and Rhodey were buddies before Tony's Iron Man campaign, and he was even by his side throughout the Civil War.
Korg is a Kronan, a rock extraterrestrial who is Thor's companion. He and Thor first met when they were in Sakaar and were forced to compete as gladiators.
Groot is a Flora colossus and has a variety of powers. Adult Groot serves as Rocket Racoon's muscle and defends the squad after their spacecraft crashes.