Premium membership is tied to your headset. To share premium, you need to logout of the app and log back in with whoever wants to play. You sign out from the Account option on the main menu.
Sharing across multiple headsets:
If you have more than one headset and would like to share premium, the premium subscriber must be the one signed into the headset with their Facebook/Oculus account and must sign into VZfit first to tie the headset to their account. After that, perform the same steps signing out and back in as described above.