I'm recalling that I registered here within the first week of the site having gone live in July of 2008. Right after Cryptic announced that they had the license to develop the game. This site used to display this classy, animated Starfleet arrowhead splash screen you had to click first. I sometimes wish they would bring that back with an animated KDF trifoil next to it.
Got the title which I do actually use on some characters, also happy with my STO T-shirt which I wear on a regular basis, and my STO map but those last two have nothing to do with the original question in the OP.
Anybody have earlier than Jul 2008 as a starting date?
Oh, you can view user profiles and input the number profile you want to view to see people's join dates in order. Doing that, you find out that the first non-cryptic person to register was this guy. If you change that 16 at the end of the URL to different numbers, you can see who joined when.