According to their official webpage, anyone can join in the herd, so long as you wear a santa suit. Includes “A coat, a hat, a beard, red pants and accessories. Gifts and candy for the kids, cash for bars and strippers.”
At their last stop before hitting the bar, I snagged a video of them chanting, “Santa needs a drink!” at the top of their lungs.
Check out a few pictures of Portland Santacon 2005 on my flickr page.