OTher than the $14.99 monthly subscription fee, are there any costs for adding music to My Music under the Apple Music Family Plan?
AppleTV 2, iOS 7.0.1
No, you can download anything available and your Family Share members can download all they want. The only limitation is storage capacity and the music being available in Music. My family uses it and we love it!
Here is a description of the Family plan if you haven't seen it.
EDIT: There could be sales tax that would be extra. I don't know how they are going to treat sales tax upon reflection...
Other than the monthly $14.99 subscription, do you have to pay for music downloaded with Apple Music Family Plan?
