Intune provides the Company Portal app out of the box. The app is available in the Windows Store. A customer would then populate it with the apps they want users to install.
If a customer is using Configuration Manager, not Intune, they can get the Intune Add-On for Configuration Manager for an additional cost (see http://www.microsoft.com/sam/en/us/intune.aspx for licensing details). They could then use the Company Portal app for internal distribution.
It is possible to write your own company portal app. MSIT built the Corporate Application Center app as a limited pilot. We would recommend that customers use the Company Portal app, but if they wanted to build their own, we could supply them with additional information on how to.
For more information, check out the following: