Your POH states you can as long as seat belts are done up on the unoccupied seat and all loose articles are secured. Providing you indicate the W+B change in the logbook prior to flight and the school/rental agency has no policy in place you should be legal. Word of warning though when carrying a pax...Brief them thoroughly. ie don't let them stack a whole bunch of ****e on the floor between their legs like ribbon, paperwork or empty camera cases
Or try this, I once took out a long time customer in a 44, took the doors out for him to take photo's, and while cruising along he get's all excited and points out the door towards a moose only to bash his arm and shoulder the rear L/H door frame due to the wind resistance, followed by some pretty severe bruising and whatnot. He's lucky he didn't dislocate his shoulder and/or tear something and I'm lucky he had nothing in his hand when he did that.