We'd definitely need reservations for Lauriol Plaza.

The MR Seminar will be held at the Arlington Central Library (in Ballston off of the Orange Line between the Ballston & Virginia Square Metro stops) - I'm thinking it would be best to go somewhere in that area that is walking distance from the library. There's the Rio Grande Cafe, The Front Page (iDine location that offers miles on Saturdays - good food & beer - they can accommodate groups), and a couple of other bars. One thing we've found for the DC happy hours is that we need to find places that are willing to take reservations for a large group.