iCloud & iCloud Photos · Sharing folders and photo libraries
Does a shared iCloud Drive folder use my storage or the other person's?
Short answer: The owner's: shared folder contents count toward the iCloud storage of whoever shared it. Downloaded copies still use space on each participant's Mac, and can be made online-only there.
The owner's. Everything in a shared folder counts toward the iCloud storage of the person who created and shared it, not the people invited. If the owner's iCloud is full, the shared folder stops syncing new changes for everyone.
On each person's Mac, though, downloaded copies of shared files use that Mac's own disk like any iCloud Drive file — and they can be made online-only to save space. See online-only iCloud files.