Once an athlete has purchased Soma NPT, they share their Soma ID with you to connect.
Athletes who purchased through iTunes must log in using the same method they originally registered with, whether that is email, Apple, or Facebook. They can also log in using their Soma ID, which appears on the home screen after purchase. An iTunes subscription is linked to that iTunes account and can only be used on the device where the purchase was made.
If you want your athletes to be able to log in from any iOS device, purchase Soma IDs for them directly through Soma Analytics rather than through iTunes.