This is just a reminder that Amazon Prime shipping discounts have always been available for up to four household members. One of those members, obviously, has to be the Prime subscriber (the one paying the bill).
To manage your Amazon Prime membership, and add new household members, go to your Amazon Account page. Under Subscriptions Management, choose Manage your Amazon Prime membership.

Here you can view Accepted invitations, as well as Invite a household member.

Make sure to take advantage of this for every household member.