As everyone know Superboy is one of the last survivors of the planet Krypton. He grew up in Smallville and had to hide his true identity, using the facade of meek Clark Kent. He didn’t have super friends until 3 teens traveled from the future to ask him to join their Super Hero Club.
Supergirl is the last survivor of Argo City, a city that was hurled into outer space with the destruction of Krypton. When the soil of Argo City transformed into Kryptonite her parents sent her to Earth to be with her cousin Kal. She too was invited to join the Legion.