(Moodle 2.9)
In Define Roles -> Allow Role Assignments, I have a Supervisor role allowed to assign a Mentor role, which exists in the User context.
When logged in as an admin, I can go to a user's profile page, click Preferences, and then Assign Roles Relative to this User, and assign a mentor to that user.
However, when logged in as a supervisor, I don't get a Preferences link, and attempts to do directly to the preferences page result in an error. I can go directly to the the Assign Roles Relative to this User page, but that's only helpful if you already know the ID of the user you want.
So it seems that in order to navigate to the Assign Roles... page, you have to be able to access the Preferences page, but having permission to assign roles doesn't automatically grant you it, and when I search for permissions that contain "preference" in the name, I don't see anything.
What is the missing piece?