To nitpick, there were at least two members with secret IDs before Sensor Girl: Dream Girl and Star Boy, who re-joined as Miss Terious and Sir Prize in ADV #350, although the masquerade lasted for all of two issues.
Mon-El might even count, on a technicality, since he was accepted in his disguise of Marvel Boy a.k.a. Legionnaire Lemon, but he was already a member at that point.