A Shared folder does not suffice to allow you to share a Photos Library. The shared folder is for sharing files, not libraries. Whenever a user writes to the photos library, the file ownership of internal files will change and you have to repair the library, before the previous user can access it again. Photos is strictly a single user application.
You can share the library between user accounts, if it is on an external drive (or in a separate partition or second internal drive), where you can enable the "Ignore Ownership on this volume" flag. Then you can use the library from a different user account. This flag will prevent the file ownership issues. Prepare the drive or partition like described here: Move your Photos library to save space on your Mac - Apple Support
But even this method will only work, if the drive is mounted, before you log into your user account and other users, who try to use Photos log off. Otherwise you cannot make the shared library your System photo Library. Your System Photo Library is always in use, as long as you are logged in. And you need to make it available, before you log into your computer. This is very inconvenient for portable computers, but an external drive would not matter that much with a desktop computer.
To use a photo library with the screensaver or to see it in the Media Browser in other applications like iTunes or Safari, you need to make it your System Photo Library. Check the Photos > Preferences > General: Use as System Photo Library. This option needs to be grayed out, when you are viewing the library in Photos. If it s not grayed out, click it. Then restart Photos and other applications, that want to use the photos.
(not interested in paying for iCloud storage)?
iCloud Storage would not help to share a photo library. You can share the iCloud storage, but the content of you cloud storage can only be shared in a limited way. iCloud Photos is for syncing the photo libraries between devices, not for sharing a library.
