I'll have to second Nate's suggestion - there is a thriving post-college a cappella scene here in DC. Depending on what you are looking for, I might have some more specific suggestions.
Two events coming up that you might want to check out -
Friday April 18, 8 pm at Holton Arms School in Bethesda
Catch the Tone Rangers, Euphonism and Skyline for Hugh's Mommapalooza.
www.tonerangers.com for details.
Saturday May 10, 1 pm - 6 pm at the Sylvan Theatre (grounds of the Wash Memorial)
There will be ten groups performing - A Cappella Showcase Chorus, BlueLine, DC Accidentals, dcVocals, Don't Quit Your Day Job, Euphonism, Skyline, the Tone Rangers, Tonic & Gin, and Venus D Minor
Not all of the groups have openings, but most are happy to talk to you and audition you for future spots! Plus it's a fun niche community

amanda
amanda@euphonism.com