Clear a path from where the treadmill is downstairs to where the treadmill will be placed upstairs so there is nothing in the way to trip on while moving such a heavy object.
Show the people that are helping move the treadmill where it will be placed upstairs so everyone knows. That way you do not need to try to direct people while holding a heavy treadmill.
Position yourselves around the treadmill. If there are two people, place the stronger of the two at the front of the treadmill because it is the heaviest part and the other person behind the treadmill. If there are four people, position one at each corner and the strongest two at the front end.
Lift the treadmill and begin slowly moving up the stairs to avoid dropping the treadmill or tripping on a stair.
Place the treadmill at the top of the stairs to rest if needed and then proceed to place it in its designated spot upstairs.