The Dynamics 365 Portal is not mentioned in the Dynamics 365 suite, but it is a very powerful tool to add to your arsenal.
The portal capabilities provide you with a resource for your various audiences to get support for issues, search knowledge, collaborate with communities, suggest new ideas, and learn more about the products and services that they own.
Interesting blog posts concerning the Dynamics 365 Portal
Using Portal Capabilities in Dynamics 365
Jesper Osgaard has written an excellent and comprehensive post on TechNet which offers a great starting point if you are new to the Dynamics 365 Portal world.
How to Give 6 million Contacts Access to your Portal
Nick Doelman has written an interesting post on how you can give 6 million existing contacts access to your portal without sending out 6 million emails.