So the website's kind of ghetto - but in a previous life, I used to have to make a lot of group transportation arrangements. Talk to Neill at Blue Ridge Charters - http://www.blueridgecharters.com/ - I used him quite a bit & his guys always were on time & were good drivers.

Also, with DC transportation options, just tell them that another company (and you can pretty much pick any one you want) was willing to do a 10% kickback to you as the organizer (most are) and ask if you can get them to match it or discount the total cost. Most are willing to work with you.