Anyone can volunteer. I have only volunteered for Marathon weekend, but I expect it is somewhat similar. Volunteer registration for that opened in mid-late summer. You could sign up to help at the expo or race day. The perks were different depending on what you signed up for. All volunteers got some appropriate article of clothing for the task (T-shirt/windbreaker). If you signed for the expo, they gave you one "point" for each hour you volunteered. 16 points gets you a park ticket (they mail it to you). If you volunteer at the 1/2 or full, you got a one day/one park ticket when you completed your shift.
Shifts start as early as 12-2 on the morning on the half (Yikes!) and can be in multiple locations--start area, bag check, finish line, course marshal, water stop, etc. I have done water stops (behind Splash Mountain twice) and course marshal (in Epcot). I like to find a shift that starts at 4:30-5 and is done around 9:30-10. Marshals usually need to be at WWoS, then they bus you to your specific spot.
Registration doesn't fill immediately, but it's always best to sign up early so you get to choose what you'd like to do. HOWEVER, one year I had signed up for course marshal at MK and my spot was changed to Epcot. They reserve the right to change locations, I guess.
Here's the link to the volunteer info at RunDisney. Just select Princess.
http://www.rundisney.com/volunteer/
I think it can be great fun and a real help to runners--and a good way to get a free ticket!
Maura