This is really a server-side piece, rather than a front end UX component.
Some of this will depend on the environment, and other parts will depend on security considerations.
On an intranet site using IIS, you could use Active Directory (AD) groups to define login domain and product access.
Security considerations will define how you set this up on an internet site. If it's imperative that users can't get access to sections where they don't belong, it'd be logical to setup a registration-by-invitation system where the user and their access is setup by an admin before they're invited to complete their account registration. Otherwise, allowing users to select their own groups at login in appropriate.
Either way, the most efficient method of directing users to the appropriate content is to have a single login form that uses server-side coding at login to redirect users to the appropriate content and set security credentials/permissions to ensure that users aren't able to access incorrect content.