To add a Admin Only field to a Users record first, navigate to

Core > People Finder and Search the User 
or
Directory, and Search the User
  1. Click on the Contact Card
  2. Click the editing pencil next to General Information
  3. In the pop up box, scroll down to the Admin Viewable only section 
  4. Select the field you are looking to add to the Users record in the dropdown
  5. Click Add Additional Field and select the checkbox, date, dropdown or input the text 
  6. Click Save