To join the Premium for Family plan, your family member must enter the same address you used to set it up. In case you left the postal code blank, your family member can just type N/A on that field to verify the address.
We also suggest clearing all browser's cache and cookies before trying again.
Let us know how it goes.
“Music acts like a magic key, to which the most tightly closed heart opens.” ― Maria Augusta von Trapp