It all started with pirates, me hearty. Arrr.
Ahem.
But it all started with a pirate site, Z-Cult FM, which had a much stronger community (forum-based) than torrent selection, as I recall. Someone (it may have been citaltras, who just today posted a link to some public domain comic torrents, so hey!) started collecting and curating torrents of public domain books, which became quite popular.
That's about when I found the site, by the way.
The owner, Serj--who gained some notoriety by speaking out after getting the Marvel cease and desist order and working with a few (DC, I think) creators to make their works available through torrents--and a few others thought it'd be fun to have direct downloads, for the days when the torrents weren't sufficiently seeded. I think Golden Age Comics was running that week, with the contents of those torrent packs as the first downloads.
When Z-Cult FM started getting increasing attention from copyright holders, the GAC forums opened and the parts of the community interested in the Golden Age and public domain side migrated over.
Later, the community started expanding and GAC's growing pains (I still get the shakes, sometimes, from the downtime...) sort of birthed DCM as we realized that there were enough people for a little friendly competition to make sense, letting us all try out different approaches. Yeah, there may have been a few loud arguments along the way, too, but that's boring. Anyway, thanks to Captain DJ and Zog on the technical side, DCM showed up pretty much overnight, too.
Zog and Aussie might have more specific recollections of the earliest GAC days, and I assume there are at least a handful of Z-Culters (Z-Cultists?) who were there longer than I was who can fill in other blanks...