Sharing Photos Library between two users on mac

It probably won't work that way - and will likely need permissions to be repaired very time a different user accesses the library.

Pretty much the only way to make this work is to put the library on an external drive correctly formatted APFS or MacOS extended (journaled), and make sure the "ignore ownership" checkbox is checked. The drive will need to be connected to the mac before either user logs on.

Fuller details:

To correctly prepare the external drive:

It must be formatted APFS or MacOS extended (journaled) (preferred for spinning drives)

It Must not be (or have been) used for time machine. (A time machine drive can be used if re-formatted)

It must have the 'ignore ownership" checkbox ticked.

It should not be on a memory stick, SD or similar.

It stands to reason that if you want any sort of reasonable performance it should be a fast drive with a fast connection.

It must also not be a network drive (a direct connection is required - eg USB or Thunderbolt). File sharing services such as iCloud Drive, Drop box, Onedrive etc are not compatible with Photos libraries.

See

https://support.apple.com/en-gb/HT201517

The method with a Shared folder does no longer work for any kind of Photo Library package (Photos, iPhoto, Aperture) - it has no longer been supported since Apple changed the behaviour of photo library, so we need write access to them and the file ownership, even if we only want to read them, ever since macOS X 10.10.7 Lion.

Photos has been designed as a single user application with a local library. Only the owner can work with it, unless the library is ion a volume, where the ignore ownership flag has been set. And the volume with the library needs to be locally mounted with a wired connection, on the same Mac where the Photos.app is running. You can share additional libraries this way. Sharing the System Photos Library requires that the library is always available for the background processes when you are logged into your user account, even if you are currently not working with Photos.

I would never share my System Photos Library anyway, so others can write to add and dump thousands of photos into it or deleted photos from it - it is much safer to share a copy of the library on a sneaker drive, where others can add their photos and copy my photos out of the library.

You could create a second partition on your Mac, partition the system volume - it does not need to be an external volume. Then you can access the library on this volume from both accounts, and Time Machine can easily backup this partition as well. It will have the added bonus, that you do not need to remember to plug the external drive in before you start up the Mac, as the second partition will always be available.

You should be able to add second partition with Disk Utility, without having to wipe the drive, as long as there is enough free storage. I have frequently added a second partition, to have a clean install of a second system, whenever I had to send my Mac to Apple for service. Then they can boost from the second volume without having to wipe my user partition. Which system version do you have installed? A second volume instead of a second partition should suffice, if your system already supports this.

This document describes how to add a second volume on the system drive: Installing macOS on a separate APFS volume – Apple Support. You would not need to install a second system, just create the second volume (if you a system on the second volume, you cannot set the "Ignore Ownership" flag).