The Harley can carry two players, as there is a sidecar that takes a further passenger. The driver has basic controls, like being able to boost the speed or use a handbrake, as the bike has no certain abilities. The passenger is allowed to wield all their weapons and look around 360 degrees freely when they are in the sidecar.
The Harley is the fastest vehicle in some maps of the Call of Duty 3 multiplayer, and therefore is useful to transport important players (e.g. flag carriers, medics). It is also very useful for getting away from enemies quickly.