The Dancer Card will still be used for Just Dance 2017. On top of that there is the Global Account that will store your information and progress.
This means that you can be playing at a friend’s house using your own Dancer Card during this session. As long as you are identified by your Global Account, you can save the high scores while on your phone to your account.
This will work cross-platform. So, it does not matter if you play, for example, with a WiiU and your friend is on a PS4, or you play on a PS4 and your friends are on Xbox One.
However, the Global Account is optional; you can still play Just Dance 2017 without it.