I joined in 2011, which you can see on my profile. I found TPT through the unofficial Wii port of TPT. It was a pretty awful port, I made some improvements privately to map features like stamps to the wiimote buttons. After that I got into playing TPT on PC and making mods, these mods caused Simon and cracker64 to notice me, so I was added as a moderator.
Other devs I can't really speak for, as they were around much before I was. We can assume most of them starting playing on Skylark's server. Skylark transferred ownership to Simon, Simon then created this website and started doing rapid development on the game. Moderators were quickly added, as were devs.
I joined as a part of Skylark's server originally. A family member had showed my the bri3d port to the PSP, then they told me about Skylark's server.
After that shutdown, I moved over to Simon's server and made a few suggestions (one of them regarding moderation). shortly afterwards, I became a moderator. Then over the years I picked up multiple hats - mostly in public relations and legal consulting.