Tennis Bethesda and Rockville http://www.meetup.com/Tennis-Bethesda-Rockville/
(N.B. - that one is at the Cabin John Tennis courts near Montgomery Mall, so that one might be a bit far, however it's also a pickup tennis format).
I have tried the 2nd and 3rd ones, and as of so far, they are both not bad - mainly pickup tennis doubles, and the people do try to even the groups out due to experience. The first group I will probably try this coming Saturday, but assuming that, like its namesake, it's at a beginner/intermediate level.
There are also other non-Meetup.com groups in other various courts, such as the Olney Sports Center at the intersection of Georgia Avenue/Emory Road and also the Wheaton Regional Park. Websites like Tennisdc.com should be very helpful in your search for players as well.