Anohter example
I flew part-time relief f/w for a hospital in N. California a few years ago. The DO allowed time-building pilots (mostly European) to become hospital "volunteers," complete with ID cards.
These volunteer pilots (interns essentially) would fly with us. On the empty (no patient) legs, the PIC could allow them to fly, make approaches, land, etc. They could log this time as PIC (ICUS). On the patient on-board legs, they would log time as SIC.
The DO had the Operations Specifications/Ops Manual modified to allow SIC's, as the aircraft were Cessna 421/425. The FAA has a very liberal definition of "employee." One does not have to be on the payroll to qualify.
These volunteers actually received all of their meals for free in the hospital cafeteria because of their "volunteer" status.
They had to provide their own lodging/transportation. I do not remember what type of visas they had, but they were not employed as far as INS purposes were concerned.