Short answer: I think that I found out about HLP by searching for mods, FREDding tips and/or investigating lore. I definitely lurked for a while before making my first post.
Long answer:
I had received FS1 as a gift from someone, never having heard of it (or Descent) before. I had always been a big fan of Star Wars, and played X-Wing / TIE Fighter / X-Wing Vs. TIE Fighter / X-Wing Alliance. I really enjoyed those games and felt like XWA was such an awesome experience... If only I could easily try building my own missions...
It actually took me a couple of years to actually try Freespace out! Even though some of the missions stressed me out (almost quit because of Playing Judas), I really liked how story-driven it was, and then I discovered this thing called FRED that made it really easy to build a mission. Like, I didn't have to hack any system files or download some unstable third-party software. The developers actually just included their mission designer for anyone to use!
Eventually I discovered that a sequel was made, but it was too late to find a retail copy. It was nowhere to be found for me, until I managed to grab a copy on a website called "Home of the Underdogs" (though have since purchased a copy via GOG). Once again, had a blast playing it, loved the story, loved the music, loved the fact that it was more transparent for modding than the X-Wing series. It's probably around this point that I seriously started to lurk on HLP, trying to find some good user-made campaigns and eagerly hoping for FS3.
My first post on HLP was in 2004 (after probably at least a year or two of lurking) asking for help on creating my own campaign. I had some fun ideas! Alas, wasn't disciplined enough to finish a small set of missions, nevermind brave enough to let people play-test them. Maybe I'll give it another go...