The phrase "extra passengers" refers to additional people who are included or involved in a situation or activity. They may not be the main or necessary participants, but they are there as an addition or supplement. Full definition
Both the sedan and the hatch offer seating for five with class - leading interior volume for extra passenger comfort and cargo - carrying capability. (driving.ca)
The last row splits and folds flat for those times when carrying cargo is more important than carrying extra passengers. (automobilemag.com)
The combination of the silky smooth V12, automatic transmission and power steering make it a fantastic grand touring car, with ample room for luggage and even an occasional extra passenger. (barrett-jackson.com)