If its set up like other old jeeps that's a drivers seat, a passenger seat, bench seat in the back (3 comfortably), far back (1 comfortably) unfortunately that's only 6 seats for 8 people. So we will have to squeeze 4 people into the back seat and 2 into the far back.
I'd say O'Connor should be in the passenger seat since he has the rifle. Johnny would be fine in the back seat or the far back.
I'd suggest this setup
Clem: front seat
O'Conner: Passenger seat
Back seat (from driver side window to passenger side window):Sammy, Fionn, Sam, and Heron (mages at windows for protection)
Far back: Johnny and Icarus (cause we are probably the biggest besides Clem and O'Conner)