I can't add a family member to my premium account

[#1]

When I send them a link, it just takes them to my family premium account

[Vasil]

Hey there!

Thanks for posting in the Community 🙂

Could you please confirm you're following the steps as described here?
Tip: In order to make sure everything goes smoothly, the future plan member's account needs to be on free before they can join the Family plan. 
Tip: Opening the link from an incognito browser session can also be of help.

Let us know how it goes!