File Sharing: How re-add the Staff group if it was removed?

I have MacOS "El Capitan" About **Sharing - File Sharing**: When a directory is added to be shared through the Shared Folders pane, then through the Users pane is possible add/remove some _Users_ and _Groups_ according our needs. It through the + and - small buttons But I did realize the following: when the Staff group is removed, is not possible re-add the same Staff group anymore. It does not appear when + is clicked and then through the small dialog window opened and Users & Groups option is selected. I tried removing and adding again the same target directory to be shared but nothing. How solve this situation? If a new or other directory is added through the Shared Folders pane, in the Users pane appears already added the Staff group such as for the previous directory case.