well as someone who has left and come again...how about "lost soul" or "temporarily insane"
Wraith
the fuzzy apparition of the departed but not forgotten
I quite like Alumnus
Meaning: An alumnus can be a former member, employee, contributor or inmate as well as a former student.
I think that covers a whole range of previous members. The inmates are the ones that were on lush 24/7 and well the students - hmmm the younger members who thought they could get away with joining LOL