"Instantly turn your WordPress installation into a private corporate intranet
WordPress is a popular platform for creating corporate intranets. The only problem is that it's built primarily for public-facing websites.
There are plenty of free plugins available to add privacy and other requirements - but you'll need to cherry-pick the functionality you need, and make sure they all work well together.
All-In-One Intranet gives you everything you need in one plugin to lock down your site and start building your intranet."