One family member creates a group on the family sharing page and invites family members.
When family members accept, it will match up all family members that are shared between those in the group and the new member.
Now for every family tree member that is shared, those in the group can see what everyone in the group has shared in that tree member's memories hub.
As long as the tab in memories hub is set to shared, everyone in the group can see what everyone else in the group who shares that family member has saved in that person's memories hub.
Example:
You and your first cousin share relations on your Mom's side of the family. Both you and your cousin have full family trees from both sides of your family. When you both join the same group for this particular side of the family you both share, only those like family members will be shared between you. So now if you go to your grandmother's node on the tree and open Memories Hub with the shared tab on and go to photos, you can both see the photos that everyone in that group has shared for your grandmother. Your aunts, uncles, cousins, parents, siblings and more can all see what everyone else in the group has shared for every section of the memories hub for that person. A family member can be passive and just view what everyone else has shared or be active and share memories of her own.