It's the Eternal Youth Spa!
From "Batman: The Animated Series"!
And it's actually run by Poison Ivy!
Orrrrrr... maybe it's that place from "Death Becomes Her"...
Orrrrrr... maybe it's aliens that replace the people who join up...
Orrrrrr... maybe it's a brainwashing cult...
Orrrrrr... maybe it's just a scam.
There, all the possible plots covered. ;]