Accessing a Photos library on another Mac
To share your Photos library across a network, you first need to decide where the library itself will be stored. Choose which computer will hold the library, then open System Preferences on that computer and enable Personal File Sharing in the Sharing pane. Details on enabling file sharing on macOS can be found on Apple’s support site at these links:
How to connect with File Sharing on your Mac
Once that’s done, go to another computer that you’d like to access the library from and go the Finder. To connect to the shared computer, follow the directions on Apple’s support site at:
Connect your Mac to shared computers and servers
When you connect to the remote machine, it’s easiest to log in with your default admin username and password, since that will let you access any hard drive on the machine, and thus let you store your photos anywhere you want on the machine. Once you’re connected, you can open up PowerPhotos and set up your machine to use the remote library. Note that Photos requires both read and write access to a library in order to open it, even if you don’t intend to actually make any changes, so make sure you log in as a user that is able to modify the library.
If you have a Photos library set up on the remote machine that you want to use on your machine, all you have to do is add the library from the network drive to PowerPhotos just like you would any other library, either by selecting File > Add Library or by dragging the library into the library list. Once the library shows up in PowerPhotos, you can access it just like any other library.
When accessing this library, the machine where the library is stored must be awake and available on the network, and the other machine must be connected to that machine via file sharing, otherwise Photos may display an error when you try to open the library. PowerPhotos will automatically attempt to connect to the remote machine when you try to open the library. The machine still must already be awake for this to work, and you’ll need to type in the appropriate username/password to connect to the machine, but this can save you from having to remember to connect to the machine manually before opening the library.
In order to access the same library from multiple Macs, both Macs must have exactly the same version of macOS (and Photos) installed. If you open a library from a Mac with an older version of macOS, Photos will ask to upgrade the library to the newer version, after which the older version of macOS will no longer be able to read it.
Note: whichever library is designated the “System library” on the Mac where the library is stored will effectively always be open by macOS in the background. Since there is no way to fully close the library in this case, your system library cannot be opened from another Mac. If you wish to access a library from more than one Mac, you must designate another library to be your system library instead. This also means there is no way to create a setup where a library is both synced with iCloud and accessible locally from multiple Macs.