Passengers age 16 years and older can get their own citation for not being buckled. See 27315(e) VC.
You as the driver are responsible for making sure everyone in your vehicle is buckled up while you are driving. See 27315(d) VC.
So If I stop you because I see your adult passenger is not wearing his/her seatbelt, I can write you a ticket for 27315(d) AND your passenger for 27315(e) VC and walk away with two cites from the one stop.
If the unbelted or unsecured passenger is under age 16 then it changes a bit. If the parent is in the vehicle, even if the parent is not driving, they get the citation. See 27360(a) and (b), as well as 27360.5(a) and (b) VC.