Software for Your Intranet
The server or servers you run will need to have the appropriate software to make the Intranet functional. The users' computers will also need to have the right client software to communicate with your servers. All the software to run an intranet can actually be acquired free from the open source choices.
Here is a breakdown of the software you may want to use to create an Intranet.
You will need to have a web server software installed and configured. If you use a Windows based Server Operating System then you could set up IIS server as the web server otherwise you can use the preferred Apache web server which is open source. Apache will run on almost any Operating System so this might be preferable especially if you want to run on a none Windows Operating System.
You may also want to have a database set up as a back-end for your web server and other information storage and session management. In this case you can use an open source database such as MariaDB or MySQL. If you want your Intranet to serve up dynamic content, you may also want to install server scripting technologies such as PHP which is also open source and free. IIS web server comes with ASP as an option.
Once all these are set up and running, all that remains is getting a competent web developer to develop the Intranet the same way they would develop a website.
You may also want to extend your Intranet and include features such as FTP to allow certain users to upload files to the server. You can use a good FTP server software such as Filezilla on Windows which also comes with GUI based cross platform client. Linux comes with a preinstalled FTP daemon called vsftpd which would just need to be configured by the developer.
If you would like to handle email within your Intranet you can use mail server software such as Sendmail. For the users you can have web based email client such as Squirrel mail or a stand alone client such as Zimbra.
Finally the Intranet is accessed through your favorite web browser by typing in the http:// followed by the IP address or the server name of the Server running the web server software.