Welcome to the Community.
When activating the Premium for Family subscription, it will be available to you immediately. You can then add your family member through this page!
You won't get charged when activating it. However, keep in mind that your billing date might come a bit earlier than usual, since Premium for Family has a higher price than the regular Premium. This is to compensate for the time you were able to use the plan without paying :)
Let us know if there's anything else you need help with. Have a great day!