Apple - Can I Share All Photos in iCloud Photo Library to Family Members?
According to this article by Serenity Caldwell of iMore, there are three ways to share photos to all your family members.
Use the "Family" album that is automatically created if you have Family Sharing turned on.
Create a shared album.
Use the same iCloud account for all family members.
Nevertheless, there doesn't seem to be a way to automatically share all your photos to your family members. You will still need to copy photos into the share album. (Unless you are all using the same iCloud account, which I didn't want to do, since each user already have their own active iCloud accounts.)
How many photos and videos can be stored in a shared album?
A shared album can hold a maximum of 5000 photos and videos combined. When you reach your limit, you must delete some photos or videos before adding new ones.
iCloud Photo Sharing FAQ
My Photo Stream and iCloud Photo Sharing limits
I realize this is not directly answering the original question, but the information seems relevant because if you have > 5k photos/videos then the answer becomes "No, you cannot share all iCloud Photo Library content with a family member".
I believe so, just share your apple acount with your family members, and under the same account, all media contents including photos would be uploaded to iCloud and shared among your family member.