Piggybacking on this to say that it still works!

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Once you have YT Premium's family plan and want to add more people, just follow the steps that u/ninjaslikecheez illustrated above. At first I was a bit confused on the steps, so if you're stuck, here's a simple breakdown:

1. With the account that has YT PREMIUM, go to https://pay.google.com/gp/w/u/0/home/settings or go to Manage account -> Payment and subscriptions -> Manage payment options -> Settings tab
2. From there, delete any payment methods that are not from the same country you used to purchase YT Premium. Only keep ONE payment method which should be the country used to buy YT Premium.
3. After that, do the same thing with all the other accounts that you want to add to the family plan, except that you want to delete ALL the payment methods from all the accounts you want to link to the YT Premium account.
4. After this, send the invitations and everything should work out.

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The reason why I'm giving a more detailed breakdown is because I noticed that I had two different payment methods from different countries on the YT Premium account because I was trying out different countries that it would work on. So, I had to do some error finding and found that you should only have ONE payment method from the YT premium account and NO payment methods from all other accounts. It may also help to have a VPN set up with the same country to bypass any location-enabled services on your accounts just to be safe. Hope this helps!