I volunteered in the guest bar through Shelter/Avalon in 2011 and 2013, I think first year I was started on another bar and they brought a few of us over due to not enough staff in guest bar. Then in 2013 I think I was just put on guest bar for all my shifts if I remember correctly.
It was busy there but not overly so and there were some people who were being paid to work as well as volunteers. It was useful to get access from pyramid to other stage for the weekend and occasionally got to talk with/serve some performers and celebrities (don't remember any major superstars coming through). Think I served Alabama shakes, Paolo nutini, Harry Enfield and Chris O'dowd.
No idea if they still do it, I also volunteered in 2014 and 2015 but never back at the guest bar!