I'm not sure that all of these are really large groups, or that they are non-profit but its at least worth checking out the links. Hope this helps.

Rob

http://www.defenders.org/
http://www.fisheries.org/
http://www.sws.org/
http://nature.org/
http://www.pheasantsforever.org/
http://www.purplemartin.org/
http://www.npca.org/flash.html
http://www.huntingmall.com/tgct/
http://www.qu.org/index.cfm
http://www.peregrinefund.org/