I have no clue for sure when I started. I was a very very active member of the Finishing That Chat forum on Sondheim.com for a few years (as a high schooler) before I found this one, if I remember correctly. I do have one time-marker for posting on ATC in Feb 2002, so I would have definitely become a member before that.
I do think I was an active member during the 2000 season, I feel confident I was here for the Wild Party. But I can't be sure.