Go to http://web.archive.org/*/http://macrumors.com and click on the date closest to your join date and take a screenshot of the window.
You could look through the archive
https://www.macrumors.com/site.php?mode=archive
However I suppose you do not see the adverts/prices and if you are a really old member it will look different.
I had actually forgotten why exactly I joined up but seeing what was on the homepage at the time
https://www.macrumors.com/2006/01/15/macworld-san-francisco-2006-rumor-wrapup-winners-and-losers/
I can now remember, I had been tracking the rumors surrounding the intel iMac release. I had been a lurker here since pre iMac G5 release and bought the iMac G5 after following the keynote coverage here (unfortunately it was a rev A and a total lemon hence me following the Intels).
I guess I can admit that this is what it looked like when I started following MacRumors...July 2001.
I resisted signing up for the forums for three-and-a-half years for fear of getting sucked into it, but I lost that battle when the original iPod shuffle was released. And here I am four years later doing it full-time.
Of all things a 2004 ipod shuffle compelled you to be an active MR member?
Hey, it was a ground-breaking product. Hard to imagine looking back at it, but it was pretty darn cool at the time. Tiny 1 GB iPod that doubled as a USB flash drive for only $150!