search for pictures of a set of people together in Photos If I have pictures in macOS Photos, and people's faces have been identified and named in the People section, how can I search for pictures where both person X and person Y appear, or where X, Y and Z appear? 

Currently, after some googling and experimentation with operators, I've only been able to search for pictures by using the name of a single person.
You can achieve the same result by creating a Smart Album, specifying multiple Person criteria with matching set to all to include by name those who must feature together in the photos (and videos) that populate the album.



You're not limited by how many people against which you can specify criteria, meaning you can search for photos that feature a specific group of friends or colleagues who all appear together.

Changing a criterion from includes to does not include allows you to exclude photos in which a specific person appears.

Unfortunately, achieving more complex search patterns that comprise conditional inclusion/exclusion criteria nested within multiple matching blocks is not possible; that is to say, you can only match either any or all of your criteria collectively.  So, whilst you can find photos in which persons X, Y AND Z all appear together, you can't easily find photos which feature either persons ［X AND Y］ OR ［X AND Z］ using a single Smart Album (you would need to go through the trouble of multiple Smart Albums and—depending on how complex your search requirements become—using the Smart Album(s) to make regular albums that can be referenced by criteria of any subsequent Smart Album to narrow its search scope.  So, complex search criteria are achievable, but are inconvenient).

Let me know if you need anything clarified furhter and I'll get back to you.