Suggestions for lunch, party of 20; Watertown, Arlington, Belmont area?
A couple of ideas:
The Chateau in Waltham might work for you, as it has a parking lot and many large tables. The food is ok at best, but a good place for a large group.
You could also try the Aegean in Watertown (across from the Arsenal Mall). It's a large Greek restaurant with room for large groups. The food there is mostly good and the service is decent enough.