Mtl Restaurant needed (with a/c of course!) for #TweetUp
20 people is quite a group for Old Montreal. You don't mention budget....
You could look into Aziatik. They have a pretty large dining room with a swanky decor, and i've seen them host Happy Hours for some office parties.
Another pricier alternative is Joe Beef.
2491 Rue Notre-Dame W, Montreal, QC H3J1N6, CA