TaskReviewer allows admins to determine if they would like users to have permission to invite guest reviewers to review files.
NOTE: This setting is applied across all review stages in all projects.
You can easily modify these permissions my going to My Account from the Browser extension, then clicking on the Team Tab on the left side of the screen (Admins and Account Owners only by default).
Users do not have permission to invite guest reviewers by default. You can enable this by toggling the Invite External Reviewers switch
Once this permission has been toggled on, the team member will be able to add guest reviewers to review stages.