According to Apple policies, in-app purchases cannot be shared: https://support.apple.com/en-us/HT203046
It means that if you have purchased the premium via an in-app purchase/subscription using your Apple ID, it cannot be shared with other users using other Apple IDs.
However, the premium account will automatically be activated on all devices linked with your Apple ID and/or PocketLists.com account.
Family Subscription (coming soon)
We understand how cool would it be to allow the entire family to use premium Pocket Lists without paying for the premium for every team member.
So later in 2019, we have a special ‘Family Subscription’ coming for Pocket Lists. It will still be powered by Apple’s in-app purchase technology, but will allow you to manage family members on the Pocket Lists side + offer some exclusive and handy teamwork features.
We cannot provide any official information on the ETA and pricing for the Family Subscription, but we will do our best to ship it in the mid-2019.