That time of year again - Office Holiday party locations (focus on NOVA)
My firm used one of the private rooms at McCormick & Schmicks in Reston a few years ago for a similarly sized group. Since this is a chain, you may actually do better at "selling" the idea.
If your co-workers are more adventurous than most, I bet Tarin Thai (Elden St. not far from Worldgate) and their modern interior would jump at the chance to do a full sell-out for a night. You'd be taking over the whole place, pretty much, and they'd need to bring in a dedicated bartender to handle the crowd.
I attended a function at the Clyde's in Tyson's that was surprisingly good. I'm generally not a big fan of the Clyde's chain, but the service was excellent, the private room was sunny and cheerful, and the food was certainly edible. Now, the function was a brunch, so it may have been easier to set up, but I thought it was a nice place for a private gathering.