Access shared album without syncing photos to icloud

You were not offered an old-style Shared Album, you were offered a newfangled Shared Library. Those are very different things and play by very different rules.

As you have learned, you cannot participate in a Shared Library unless you are already using iCloud Photos, which requires enough iCloud Storage for all of the photos and videos on your own device(s).

Shared Albums, on the other hand, do not count against iCloud Storage at all, but they have other limitations on number and size of items stored.

Personally, I use various Shared Albums for sharing vacation and events photos with different groups of friends and family members, saving my one and only Shared Library for longer-term collaboration with my spouse.

If your family member has analyzed the differences and decided that a Shared Library is more appropriate for this project, you'll have to do your own analysis to decide whether the cost is worth the benefit.