One of our members, Matthew, mentioned that the distinguished lecture series of the Mathematics Association of America holds events locally.
The next such lecture (at the Carriage House Conference Center, Wash, D.C.) takes place on Thursday, September 23 at 6:30 PM.
Lecturer: Annalisa Crannell, Franklin & Marshall College
The details are at: http://www.maa.org/dist-lecture/
This is not an "official" meetup event, but I plan to attend, and I imagine some of you out there will be interested in attending as well.