You must be logged in as the person who purchased the subscription. (Don't know who that is? On your family page, you can see the primary subscriber -it will say "Subscriber" under their name.)

From the hamburger menu in the top left, choose "My Family" then "Manage my family".

On desktop: to the right of the main subscriber's name, choose the tree dots. On mobile: tap the main subscriber's name.

Choose "Subscriber permissions".

Assign the subscription to the family member you want to give access to by checking the box next to their name.