The team should be aware of the "faces" branch in Shotwell's master repo. A great deal of good work was performed by Valentin Barros to add face support to Shotwell. This involved writing a major UI tool (drawing boundaries on faces, typing in a name, autocompletion) and integration with a face recognition library.
However, the branch lacks certain integrating features, like the ability to connect a face to an address book or social network. Also, at the time of coding, the recognition software left something to be desired (not accurate enough).
If this is desireable for Photos, a good place to start would be the faces branch on Shotwell's git repo. I should add that this is by far the most requested feature missing in Shotwell today.
A listing of bugs filed against the branch: https://bugzilla.gnome.org/buglist.cgi?product=shotwell&bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED&component=faces
The branch itself: https://git.gnome.org/browse/shotwell/log/?h=faces