iCloud & iCloud Photos · Sharing folders and photo libraries
Does iCloud Shared Photo Library use my iCloud storage?
Short answer: It uses only the iCloud storage of the person who created it; participants add photos without using their own space. On each Mac the photos still take local disk space, which Optimize Mac Storage can reduce.
Only the storage of the person who created the Shared Library. Everyone else can add to it without using their own iCloud space. If the creator's iCloud fills up, new items stop syncing to the Shared Library for everyone.
On each Mac, shared photos also take local space like the rest of your library; Optimize Mac Storage in Photos settings keeps smaller versions to save room. See reducing the space Photos uses.