Hey there,
Thanks for coming to the Community.
It sounds like all family members are trying to use the same Spotify account to listen to music at the same time. To be able to listen to music at the same time, all family members need to have their own Spotify account. If they don't have a Spotify account yet, they can sign up at http://spoti.fi/2e1FH6k.
You can learn how to invite family members to your Premium for Family plan here: https://support.spotify.com/article/invite-remove-family-member/.
Hope this helps 🙂
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