Here are some van rental companies in the DC area from which you can rent a large size ( 12- or 15-person capacity) van to take a group to a protest, training, conference, or other event. Generally, you have to be 25 years old to rent a passenger van. Keep in mind that there are generally fees and taxes above the daily rate, such as insurance and fees for going above a certain mileage. Be sure to check the companies' websites for up-to-date information about prices and availability. (updated summer 2011)

image from www.flickr.com/photos/jagspace/2950936886/

Budget:
Locations - Union Station, Ronald Reagan Airport, Dulles Airport, and various other locations in the area (check website)
Budget has 12-passenger vans at $180.49/day plus other fees.

Avis:
Locations - Union Station, Dulles Airport, Ronald Reagan Airport, and various other locations in the area (check website)
Avis has 12-passenger vans at $75.99/day plus other fees.

Apple Car and Truck Rental:
Locations - D.c., Alexandria, and lots of other locations in MD and VA (check website)
They have 15-passenger vans for: $165.99 per day with 200 free miles per day, $179.99 per day with 350 free miles per day.

Image Rent-A-Car:
They are located in the NYC area, but will deliver a van in the DC area.
They have a wide selection of vans, anywhere from 8- to 15-passenger vans.
8-10-12 passenger vans are generally $129-$189 per day, depending on the time of year, and 15-passenger vans are a little more expensive.

Rent-A-Wreck:
Their Falls Church location has 15-passenger vans at $129.99 per day.