Former classic SmartMusic users need to create a new account for the new SmartMusic. Classic SmartMusic and the new SmartMusic are entirely separate platforms, and your classic SmartMusic account does not carry over to the new SmartMusic. Since there is no shared data between the two platforms, you can use the same email address and username you used in classic SmartMusic, as long as it's not already in use in the new SmartMusic.
If you were invited to connect to your SmartMusic organization in the role of Platform Owner or Administrator, this article has instructions for getting started. If you were invited to connect in the role of Teacher, instructions for getting started can be found in this article instead. If you are both an Administrator and a Teacher, you need to make yourself a teacher after you have connected as an Administrator.