I have a few reviewer licenses that need to have access to a Portfolio. How do I give a reviewer access to a Portfolio?

According to the "Access to objects and areas by license type" article on Workfront One, the Review license does allow for View access to a portfolio. Since the license type does allow view access, you'd need to grant that through the access level. And then you'll need to give sharing permissions on the portfolio itself.

I find the "Configurable access to functionality for each object type" article to be helpful, too.