You can add users with the " Reviewer " role from the People's section of your Organization dashboard. (Only the Owner, admins, and contributors can add a Reviewer to the organization)
From the “Invite people” modal window, you get to choose the role of the invitees. Select the role of Reviewers and add the email addresses of the people you want to add. You do have the option to bulk-add Reviewers by using comma-separated email addresses.
From the context menu, you can “Remove” a user as a Reviewer from your Organization.