At some stage, most companies discover that due to their increasing size its hard to keep track of what everyone is doing. Therefore, they will require some form of Intranet to be implemented. Obviously this happened at a company that I was working at. As such, we had to evaluate the many options available to us available at the time. Obviously, there are various classes/types of CMS's. Personal ones favour a flat file structure with no multi-user functionality while the more complex/professional systems use database backends, a web based front end, have the ability to authenticate via a network directory system and have many options with regards to add-ons which allow for extra functionality.
One thing you'll need to watch for is whether there is too much of a learning curve in using the application itself. I noticed this myself when using the more mature systems such as Joomla. In such cases, either user training is going to be required and/or the content may need to be managed by your IT team. For mid-sized companies Drupal is a decent compromise between scalability and functionality.
Installation is fairly simple and similar to other web based applications. Install/untar the packages. Configure your web server. Setup the database backend. Configure the relevant files in order to authenticate correctly via the relevant backend. Thereafter, add any extra functionality as and when required.