such as SUPERBOY the original one not the one after the Superman/ Doomsday saga.
Superman was never Superboy in this continuity. He did meet up with a Superboy who was basically the "pre-Crisis" version, who was described as living in an alternate universe.
Wasn't Superman always a member of the JLA .. why do they mention that he joins and rejoins JLA ?
I think he quit at the start of the Giffen era (wouldn't you?) or maybe even before that, and came back when Jurgens started doing JLA.