I'm not sure how large of a group you are looking for, but here is the hub for the various events on the Mount.
http://www.cruisenh.com/privateevents.php
It looks like you can charter the whole boat, it can do seated meals for 500 people and reception style for 800-1,000. There looks to be some rooms available that can accommodate 125-200 people
I've included some links to some of the detailed information. It contradicts itself a little bit, but I would recommend contacting their offices to get more information.
http://www.cruisenh.com/charter.php
http://www.cruisenh.com/floor.php
http://www.cruisenh.com/ReceptionRooms.php
I got married on the Mount in July '14 in the Flagship Lounge. Guest count was 115 and it didn't feel that crowded at all. Everyone says they had a great time.