Users have the ability to hide themselves from the Directory if the 'Allow users to remove their name from the online directory' check box is checked under 'Profile Publish Access.' 

To stop Users from hiding themselves:

From Core, hover over Settings and click Profile. Then click on Profile Publish Access

  • Edit and uncheck box next to 'Allow users to remove their name from the online directory'
  • click Save

Users will now be unable to hide their name from the Directory.