On payload maps, the payload can reverse direction and move back towards the start. According to the Overwatch wiki (http://overwatch.gamepedia.com/Escort), "The payload will roll back towards the start if no attacking players are near it." However, I have often heard defenders on streams say "I'm going to get on the payload to move it back." Is this necessary? Or are they just confused?
The payload will start moving backwards on defense whether or not anyone on the defending team is riding it. It will start moving backwards 10 seconds after the offense leaves its zone of effect. (Source). It does not gain a speed benefit from having anyone stand on it.
When attacking, it gets a speed boost for every person standing on or near it, up to three people. The 4th person and beyond will not give it any more of a speed increase. Note that this is true when capturing an objective as well.
Whether attacking or defending, the payload will not move (in either direction) if there is an enemy on it contesting.