Issue Summary

Currently, there is no permission toggle that allows users to Add/Edit Members in a Portfolio without also being able to edit the portfolio itself

The same behavior can be seen in the Program.

Suggestion

I would like to suggest more granularity for Portfolios permissions toggle:

Portfolio -> Manage -> Portfolio teams: “This is a read-only view. If you want to allow updates then you will need to toggle on the Portfolios Permission under Administration Toggles.”

That allows a user role to see and access the Portfolio details (read-only) but also able to Add/Edit Members tab 
Since it also can be seen in Program -> Manage -> Program teams please extend this suggestion to the Program level too.