Pretty much every worker there is a volunteer. I've hung out with a few of them and they all say the same thing: there are good and bad things to this.
You get a free ticket, but you don't get to choose which events you get to go see or what shifts you work, so you may end up missing the shows you want to see. You are able to switch shifts with other volunteers though, so yo may be able to see everyone you want to see. You also don't get a choice on what job you get either. Could be security, could be garbage cleanup.
And don't get any ideas of skipping your volunteering duties. You pay for your ticket up front, and are given the money back at the end of the weekend, provided you didn't bail on the gig.