How do I handle "customers of customers" in Studio and the API?

An Organization represents the paying customer. It is a 1:1 relationship between the contractual owner of a sensor or Cloud Connector and our Studio or API. 
Representing "customers of customers" is best done on the Project level in Studio. It is possible for the Organization Admin to create new Projects both in Studio and the API.
Please see the Getting started with Studio guide for more information on the general concepts of Studio and the API.