Jack up here in Boston. I'd say their best bet, for one, would be to join the SSE facebook page here:
https://www.facebook.com/groups/SeaSoundersEast/Kent Straub-Jones, or "kent" known here on the boards, is a local and also one of the admins with me on the SSE page. He usually will post meet-ups for the DC area on there.
My girlfriend is a Baltimore native and I sometimes will make my way down there as well; plan is to eventually move down that way so I may join them!