The only place I'm aware of is Brewster Academy, which has massive grounds with magnificent views directly on the Lake and could accommodate a tent with a crowd the size you have in mind. Link with photos and contact info:
http://www.brewsteracademy.org/RelId...t_Brewster.htm
There are also apparently large private manses on the Lake that will rent out their grounds for weddings and such, but you might have to go through a Wedding Planner for these venues because I suspect the owners would not want the hassle and risk of dealing with individuals. Here's a Wedding Planner site that claims access to such places (I have no knowledge of this company other than what they say on their website):
http://www.signatureeventsnh.com/weddings/index.htm
Congratulations, good luck, and if you find something please share here because folks often have questions about wedding sites.