What is the best option for an intranet?

I started this topic two days ago, but I didn't get my questions answered, so I'm trying again. Even though I'm using Multisite WP, I'm using it just for one website. My idea is to have the main site, then create another site to be an intranet, password protected, just for the members of the religious congregation for whom I'm building the site. In that internal part we would have forums, activity calendars, documents, etc. I think it's better than to have two separate WP installations. I installed bbpress on the main site just to try it, but I'd need it to be password protected, not open to the public. I also looked at P2 theme... BuddyPress seems like too difficult for me... What is the best way to do this? Can you give me any recommendations?