Does family sharing cost anything?

No. The only thing that costs extra are if you take out certain family level subscriptions. For example, if you want to share a higher level of iCloud storage you need a family level plan. Plain Family Sharing is free.

To learn how to 'Set up Family Sharing', click here --> Set up Family Sharing - Apple Support

While there is no additional cost per se, if you are the organizer then you are undertaking to pay for the purchases by other family members that can't be covered by their store credit. May not be relevant in your case but I've heard of groups of friends setting up family sharing so they can share content without understanding the implications.

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