Encino for large group
I know it's a chain, but we had our office xmas party at Buca Di beppo in Encino.
We reserved the 'Pope table' which I believe sits about 15 people. It was like a private room with the pope's head in a lucite box in the center of the table. The meal was served family style and unlike Maggiano's, you can pick as little or as much as you want. I was very impressed with the service and the food ranged from acceptable to good. Everyone in our party enjoyed the atmosphere and the food.
Sisley might be a very good idea w/ that back room (I like the grilled veggie appetizer and chopped salad w/ eggplant).
Maybe consider going over to Sherman Oaks to Stanley's.
Cafe Cordiale in Sherman Oaks can easily accomodate and has a pretty wide menu.
Marmalade has a back room that can seat you.
Tempo and Rio could both work.
Yamato would be good if you'd like to take up an entire teppanyaki table.
Alcazar on Ventura between Louise and Balboa for Lebanese is perfect for a large group. The dining room is huge (and there's also an outdoor patio). You can order a bunch of mezza -- the Lebanese equivalent of tapas -- to share. It also fits the more casual variety, but is definitely a sit-down place.