Passengers on training flights are sometimes welcome, especially when trying to learn the student the effect additional weight and C.G. shift has on aircraft's flying ability (while remaining withing limits of course) or to check if student is able to do a passenger briefing (emergencies, evacuation, sick bags, use of seatbelts, maintaining "sterile" cockpit when in traffic pattern due to ATC, etc.). All good and well, as long as passengers aren't paying for flight.
On another topic (not so far away), is person with pilot's license (and rating, medical, etc.) occupying a pilot's seat in a single-pilot aircraft legally a passenger or a pilot? For example, if you don't have 3 landings in the last 90 days, can you take a friend pilot (which is current for carrying passengers) with you or do you have to do the remaining landings alone/with another FI?