Check out the budweiser plant biergarden. They host it out for private venues. It's a nice spot.

Also if you're in Southern NH and willing to cross into Tewksbery there's Wamesit lanes. They got a private section with a few lanes you can reserve for parties.

Boston Billiards in Nashua also had a private room. 40 people would be extreme, and last I checked they recently rebranded into a casino instead of billiards so that might not be your thing.