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.