If the aircraft is Experimental in any subcategory, the FAA issues a Letter of Limitations that usually says that you cannot carry passengers for hire and will likely state that only persons incdent to the flight test may be carried. However, there are subsets of Experimental such as Amateur Built, R&D, Exhibition, Market Survey, each having similar but a different set of limitations. For instance, under Experimental - Exhibition motion picture support is allowed, including carrying extra, actors, and stunt people. Just don't carry them to and from the movie set or give joy rides.
Been there, done that.
Aircraft in Restricted Category also carry letters of limitations.
Never a good idea to violate the letter, for safety of legal reasons.
We have a pretty simple set of rules here and we brief all persons involved in the test to include potential risks. That usually keeps the hangar-on types away.