When did you join?
I joined in October 1999, while in college studying Computer Science. I was also working as a developer at a local gambling games company at the time, and learning DirectX (mainly DirectDraw).
What brought you here?
I don't actually remember how I found the site, but I do remember sometime in my first year or so, me and a coworker of mine discovered an article "Why plugable factories rock my multiplayer world!" that completely took me to the next level as a programmer. (the article didn't do it all by itself, but it kicked off a sequence of learning and trying things over the next 6 months that was the most learning and growth I've had in a similar period ever. I also really enjoyed the NeHe articles on OpenGL which I used to learn OpenGL for a job a few years later.
Why do you participate?
I don't really participate much currently. I've been in corperate america developer gigs since about 2003, when I switched from C++ and embedded systems to C# and web based / service oriented software development. I maintained my game dev / C++ skills for another 4-5 years after that, but once I had my daughter I simply haven't had enough time to keep up with all of my hobbies and passions. I have however learned a little unity 3d because a friend of mine now works in the industry, and in fact participated in a game jam a few years back, and just this year built a clone of an old bar top game "color invasion". As for why ... the participation I've been doing since 2002 is almost all just to enjoy interacting with people, and mainly to help answer questions for beginners and intermediate developers. Although I do find the brand new member's constant repeated questions sometimes frustrating, I love when someone is just on the cusp of "getting it" and you can give them a few insights into how to level up.