According to the family help guide here:
What if I want to add another subaccount to my plan?
The number of subaccounts (i.e. the number of people you can invite) that you selected during signup cannot be changed. You may however cancel the subscription – at which point all members of the plan revert to Free level subscriptions if the current billing month lapses. You would then sign up again, selecting the number of subaccounts you want. You may re-invite the people you want to keep on the plan.
So you would have to cancel your current family plan and sign up again.
If you have any questions or you need help,
I suggest you contact support so they can help you
If you get an automated reply email telling you to check the help section or the community, you need to reply back to it.
Support usually replies within 24-48 hours.