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Each family member has to have a separate username and password. So the first step is to create a new account for your daughter (if you haven't already).
You can create it here:
Send her the invite to the email you used to create her account. (More detailed instructions on how to do that can be found on the FAQ)
Accept the invite and you'll be set. Both will be able to listen to music at the same time, and will have separate libraries, playlists and friends.