You can send invitations to your family if you log into www.storytel.com, go to My pages > Invite family member and enter their email address. They will be sent an activation link and by accepting the invitation they will be directed to a signup page.
They can be an existing user or they can create a new account. If they already subscribe to Storytel, they need to use the same log in details as their current subscription is connected to and their subscription is cancelled once they accept the invitation to your Family plan.
Keep in mind: It's not possible to join a family plan if the person you want to invite has a current subscription that is paid through in app payment. Before joining your Family or Family Plus subscription, they need to first terminate their subscription with in app payment and wait until it's completely cancelled before joining your family plan.