Anytime I go to Washington, especially with a group, the Marrakesh is a must-visit. They are very accommodating, even for groups of forty, with prior reservations.
The prix fixe menu is excellent - the seven courses include the most amazing lemon chicken known to man (it falls off the bones, and I've sat with people poking between the bones trying to find just one more succulent bite). I'm not a fan of the minty tea at the end (tastes like my toothpaste) but I have friends who gladly take it off my hands).
As with many such restaurants, soft drink refills (on those small, small glasses) are not free. Stick with water as your base beverage.