Thank you for reaching out to the Community and welcome. We moved your post from Chats to the help boards as its content fits better here.
Keep in mind that the official language used in the Community is English; the replies will be posted in this language. We appreciate your understanding.
On Spotify, an account can only change the plan once every 12 months. This means that if the other user was already in 2 group plans before (Family/Duo), it cannot join a third plan within the 12 months.
Keep in mind that the time to join a third plan is 12 months ahead from the date that the secondary plan started.
However, since this type of issue is account-related, we'd recommend contacting our Customer Support so they can check your accounts to see what is happening.
Hope this helps. If anything else comes up, the Community will be here for you.
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