Megan,
You might try going to the local sessions. This way you can meet musicians and check them out. Maybe you'll like some of them, their personalities and their playing and put a band together. Or maybe you'll meet some other singers. You could start an a cappella band.

Getting out and mixing with the players is the best way. There is the Revels that always is looking for singers (at least in Houston it is). You'll meet a bunch a fun people.

Finding people to do music with is just like finding a significant other. You get out of the house, start talking to people, express your desires, meet their friends, etc.

When I think back on how I got into the various groups I've been in, its amazing all the different connections. I was in a great a cappella group that I heard audition announcements about while singing in a small church choir. Was doing the church choir only to keep my sight reading skills up. Got into another group by way of a folk dancing group I was in.

Kat's idea with the link is a good idea. I've always thought of the DC folk scene as thriving. Surely there is room for another singer. You just may have to make something happen yourself.
Good luck. Keep us posted on your progress.

Emily B