Shared library not working for all invitees

Let's start with the second warning. Were you surprised that she already owned (or was connected to) a Shared Library, do you not think that is true, or did you know that a user can only own or be connected to one Shared Library at a time?

The first case is more complex. The message he got doesn't help, and you may be misinterpreting it.

Your friend would need more space on his iPhone to join your Shared Library. Every photo or video in a library (Shared or normal) connected to a phone requires space on that phone. If the phone has Optimize iPhone Storage enabled, the storage needed on the phone may be as little as the space for a minimal thumbnail and some indexing; if not, room for the full resolution photo or video is required.

What less clear is why your friend was told he needed more iCloud space. Is it possible that he had not been using iCloud Photos previously? It is a prerequisite to Shared Library to have an iCloud Photos library of your own, and enabling iCloud Photos for the first time could require adding iCloud storage to upload his own photos and videos.

I tested the shared library by sharing my library with myself, right after Apple first introduced it with macOS Ventura in 2022.

At that time I noticed a huge increase of used iCloud Storage when I joined the Shared Library using my Apple account I use for work. The reason has been that iCloud created automatically a complete safety copy of my current iCloud Photos Library which has been kept for six months. Apple has reduced this time to six weeks now:

Create or join an iCloud Shared Photo Library in Photos on Mac - Apple Support (EG)

When you join a Shared Library, iCloud captures a one-time status of your photo library database and metadata and stores it for up to 90 days. This helps with a smooth transition to iCloud Shared Photo Library. Any photos that were in your library at the time of the snapshot that are subsequently deleted are stored for up to 90 days.

Perhaps this safety copy snapshot is the reason for the additional iCloud storage required when joining a shared library. We need enough iCloud storage for our personal library, including the backup copy, before we can join the shared library. The first few month after joining the shared iCloud Photos Library it has not been possible to reduce the amount of iCloud storage used by my personal iCloud Photos Library. Photos I deleted to free cloud storage remained in the snapshot.

Your screenshots explain it. We can only join a Shared iCloud Photos Library if we already are using iCloud Photos. We are sharing our photos from our iCloud Photos Library to the Shared iCloud Photos Library.

On a Mac that would be no problem. Here the invitees could make a new, empty library their System Photos Library and turn on iCloud Photos for it. Then the free 5GB Storage should suffice. They would just have to move any new photos they want to share from their previous library to their personal iCloud Library and move them from there to the Shared iCloud Photos Library, always in small groups, so the personal library remains small and they need not much storage. But on the mobile devices we cannot have several libraries like on our Macs. There the invitees will need enough iCloud storage to keep the all their photos from the personal library in iCloud. If the people you invited are your family members, you could invite them to Family Sharing to share the iCloud+ subscription also for the storage for the personal library.

There is no view-only option for a Shared Library. If you subscribe, you share the library fully with the owner. (The only exception that comes to mind is that you cannot invite more subscribers.)

As I said, even with maximum optimization, the subscriber's phone would need to keep indexing information and minimal thumbnails for all of the items in the Shared Library so that it could display them when offline.

Addendum:

After rereading the rules for Shared Library one more time, I see a reason why a subscriber might need to add iCloud to join, to cover the possibility of the owner deleting the Shared Library later. "After you delete the Shared Library, all participants receive a notification. If a participant has been in the Shared Library for more than seven days, they automatically receive everything from the Shared Library in their Personal Library." [from How to use iCloud Shared Photo Library - Apple Support] That might explain the message if it really was suggesting (or demanding) more iCloud storage.

But the iCloud storage for the Shared Library should be provided by the organizer, mark. Otherwise this message would be very misleading:

However, we may need plenty of additional storage on our devices, just in case someone will flood the shared library with thousands of photos. There will be no polite way from keeping them to from drowning our devices with photos we do not want to see or using up all our iCloud storage. A pity, that Apple does not offer an option to set up a storage quota for each subscriber.

I agree that the storage should be provided by the owner, léonie. I even thought it would be provided from the owner's quota, and that has been my (limited) experience. The current post has cast some doubt on that, and we are trying to work through why that is. Maybe, if and when we get a screenshot of the exact message, that will help.

The text in the red box in your screenshot almost certainly just ignores the possibility of the subscribers some day inheriting the full contents of the Shared Library as mentioned in the text I quoted.

Sorry for the side chatter, Duchy777.

As to the Shared Albums instead of the Shared Library:

On the plus side:

• The cloud storage for shared albums is free.
• And you are sharing the photos organized in albums. In the Shared Library you are only sharing the videos and photos without any organization, the albums and folders are not shared.
• The Shared photos are shared as copies, and nobody can change them. So your own photos are not put at risk when sharing.
• You don't need to turn on iCloud Photos.

On the minus side:

• Photos in shared albums are not shared in the full quality. They will be downsized to 2048 pixel on the longest edge.
• Depending on the system version some metadata will be stripped for privacy reasons.
• There are limits to the number of shared albums, the number of items in the shared albums, and the number of uploads per day. iCloud: My Photo Stream and iCloud Photo Sharing limits