Should participates be email address or phone number or does it matter?

Hi georgiadw,

Thank you for using the Apple Support Communities! We see you were linked from the article "How to use iCloud Shared Photo Library". iCloud Shared Photo Library would use iCloud Photos as noted in the same article:

To join a Shared Library, you need a device with iOS 16.1, iPadOS 16.1, or macOS Ventura or later with iCloud Photos turned on. If you're under 13, you can only join or create a Shared Library with members of your Family Sharing group.

Hope this helps!

Have a great day.