On the web, go to Feed and hover over the feed in the list.
Click the pencil. The audience section shows how many members the feed has, plus the roles and locations it targets.
Click Edit audience to change the roles and locations. The count reads Visible to [n] employees.
Click Manually adjust audience to see who was added or removed by hand, under Included and Excluded. Search for someone and click Include or Exclude to change it.
Click Save in the window, then Save again on the audience.
Only Admins and Editors can open a feed's settings.