In order to help you Tag faces on previously uploaded photos, we have added a new sort category "Face Detection Candidates" in the Photo page of each Album. You need to be signed in as Administrator to use this feature.
[1] Log in as Administrator
[2] Pick "Photos" under "Media" in the menu.
[3] Click on an Album.
[4] Choose "Face Detection Candidates" in the sort selection.
[5] Click "edit" on any photos photos that have the Tag icon.