I found out about SRB2 on Easter 2008 when me and my cousin were bored and started browsing for freeware games on the website of a German computer magazine. We were originally looking for Super Mario stuff, but when we ran out of things to play, I remembered that there was this other videogame character, "like a hedgehog or something". The only thing we found was SRB2. Because 1.09.4 still had these stupid date-activated secrets and because we had never played a Sonic game before, we thought at first that the goal of the game was to find the easter eggs. We must have spent around ten minutes in GFZ1 going in circles because at this point the star post was more or less obscured by a GFZROCK wall. Eventually we got to the end of the level, realized that easter eggs weren't the goal of the game and got to THZ1 (I think) before the day was over.
My cousin quickly lost interest and never picked the game up again, but I was fascinated and played it again and again, unable to progress beyond the first section of THZ2 for several weeks. I eventually got a little better, finished the game, went to their website and found out that you could download and play custom levels (a good portion of which were then hosted on the main website in a dedicated download section). I also discovered the forum and lurked around there for a while until it was November and SSNTails announced there would be a new version of SRB2 coming out in May. At that point I knew I was going to stay.
I eventually registered a forum account in March 2009 but stopped posting after a few days for some reason. I was around when the shitty April Fools' prank with the "n00bish" board happened, but I don't think I participated. I started posting again in May and got a lot more active when 2.0 was finally released. It was originally slated for July 25th, the day I was going to do on a two-week vacation with no internet access. Luckily for me, SSNTails released it a day earlier, so I played 2.0 the whole night through and spent the subsequent vacation being an annoying brat and wishing I was at home playing SRB2. Since then I've been a regular here, hanging around on IRC all the time and performing various tasks in the community, like moderating this very message board.
tl;dr: known about it for five years, been active for four, and 1.09.4 was the first version I played. If you want to see some real oldbies, look out for people who have "Nov 2003" as their join date: That was when the old message board crashed and they migrated to this one instead, so most of these people have been here for more than 10 years now.