Sharing a library between multiple user accounts on one Mac

Photos is not designed with multi-user use in mind, which makes it very difficult to create a setup where a single Photos library can be accessed from multiple accounts on your Mac. If you wish to share a library this way, the following restrictions will apply (for the below discussion, assume we have two users named A and B that we want to share a library):

1. Only one user account can ever have a given library open in Photos at a time. Before you can use the shared library as user A, you must first make sure to quit Photos as user B.
2. None of the user accounts that are accessing the library can designate it as their “system library”. The system library is effectively open at all times by macOS in the background as long as that user is logged in, so even quitting Photos will not free up the library so that another user can open it. You must either designate a different library as the system library, or fully log out user B before trying to access the library as user A. Note that this also precludes having this library sync with iCloud Photos.
3. The library cannot be stored on your internal drive, or any other drive where permissions are respected. Fighting against permissions on macOS is a losing battle; while you can play whack-a-mole trying to fix permissions on the library before you open it, something will eventually go awry and it’s likely you’ll be unable to access some portion of your library. You should store the library on an external drive, and in the Get Info window in the Finder, make sure that the “Ignore ownership on this volume” checkbox is checked.