Amazon now offers Prime Members: Family Vault which allows you to share Unlimited Photo Storage w/ up to 5 people. Thanks okinawanmatt

Note: Videos and other files can be stored too, but storage is capped at 5GB across those five additional users. From Prime Photos, you can send invites to 5 people to join your Family Vault via Desktop or App (iOS, Android). Once added, each Family Vault member can contribute to the group's photo archive.

https://www.amazon.com/photos/home?tag=