There is a ridesharing app and website named PoolXing - www.poolxing.com for finding and connecting with carpoolers in the Washington DC-VA-MD metropolitan area.
The more the people start using it, the more useful it will get.
I am the founder of the company and currently there are more than 70 users in DC Metro area. There are new users signing every week. Though it is not possible to find how many of them are sluggers. The more the users, the more the likelihood of finding someone to carpool/slug with.
We intend to incorporate more features in the our forthcoming updates that will make the app and website even more useful to sluggers and carpoolers.