For those wanting to get started on this before it’s added to PhotoStructure, two software solutions I found are:
TagThatPhoto / Tag That Photo is Windows-only and charges a subscription to recognize more than 10 people. It uses commercial face recognition software from Applied Recognition which review say is quite good, and you can also use some peer-to-peer stuff to collaborate on tagging with others.
digiKam is free, open-source, and cross-platform (Windows, macOS, Linux, and BSD). But I guess the open-source facial recognition libraries aren’t as good yet as commercial ones.
Both allow you to manually tag people who aren’t recognized, for example if you can only see the back of their head they aren’t automatically recognized. You could of course just add hierarchical tags for names of people in the photos, but I recommend using the manual tagging feature to add names for particular region of photos, so that extended family/friends/descendants can still know exactly which person is connected to which name.
Also see related forum thread Facial Recognition / Additional Photos / Google Takeout