If I understand correctly, you are talking about possibly entering into a limited partnership with others who you may not really know all that well.
For myself personally, there is no way I'd do such a thing.
Has the owner stated that the site is for sale?
I'd deal with him directly and see what he would settle for. Be sure you get ownership of the domain name and any copyrighted logos as well as the contents of the forum.
Lots of variables that will affect the value of the site.
