When you start a Family plan everyone gets to keep their own account with their personal log in info and saved music. You won't share one account, you'll need to accept your invitation from your existing account to be part of the subscription.
The service you get with the Family plan has the same features as the Premium Individual plus some more, like for example a Family mix playlist, at a discounted price - which means that you'll still get to listen to your tunes at the highest possible quality.
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