I think this also depends on your price point. My work frequently takes executives to the Oar House over near the Decks....and the food is great...assuming you are pro-Seafood. If you reserve ahead of time, and ask for a quiet space on the top floor, they'll usually do what they can to accomodate you.
Jumpin' Jays Fish Cafe really has their act together - I think with advanced warning they could accommodate 14 people. Great fish, sides, etc. - not overly loud or tacky, nor "too fancy." It's where I'd host a business dinner in Portsmouth.
As backup, I'd actually consider Portsmouth Brewery - great, highly renowned beer, and surprisingly good food. Vibrant setting and very large - again, seems like it could easily find a place to stash 14 people. Louder than Jumpin Jays.
150 Congress Street Portsmouth, NH 03801
56 Market St
Portsmouth, NH 03801