Keith,
Michele is correct on the standard way to get user and patient information from the SMART standard, and these are supported in our implementation.
For the access question: Yes, assuming you're using user/... scopes, you would have access to the patients the physician has access to. However, if using a user/... scope, there is no patient in context - your app would need to write its own search anyway for the practitioner. The workflow used often depends on where your app launches from/if it is launched or standalone.
Regards,
~ Jenni