To set up a Shared Library, you need a device with iOS 16.1, iPadOS 16.1, or macOS Ventura or later with iCloud Photos turned on. For those under 13, they can only join or create a Shared Library with members of their Family Sharing group.
How to create a shared library
Go to Settings > Photos, then tap Shared Library
Choose up to five people to invite, or you can invite participants later
Choose past photos and videos that you want to move to your Shared Library, or choose to move photos later
Follow the onscreen steps to preview your Shared Library, confirm the content that you want to share, and invite additional participants