The closest one is CAMOM (columbia area mothers of multiples). Because it is the only active one in the area, it has many members from baltimore county and baltimore city and some members from Harford county. There is a very active online forum, so even if you cant make it to events, you can still get lots of support. The website is www.camom.net
I am a member of CAMOM and live in the city as do several of the moms. There is also a group in Anne Arundel County, I am not sure if that is closer for you.
Re: MoMs groups?