You want more volonteers!
Do you want more volunteers! So many that you have to turn some down? Someone mentioned hiring outside help? Why pay others? Reward our own by free access. Small price to pay, during these hard times and rough economy.
Why not offer free registration for a designated amount of volunteered time? Perhaps for a set amount of hours or the task at hand?
Concert security get free access to the concert they work, the organizers would never ask them to pay to enter and then ask to volunteer. They hire the clean up crew!