Depending on the age of machine. Let's start with a PR-450 The operator, steers, controls conveyor, starts and stop milling drum. Turns on hydromatic grade controls, and changes slope is running on slope. Raises and lowers all legs, to get drum to clear obstacles or get drum close to surface to be milled. Also controls the moldboard raising and lowering. Also controls water flow and on and off. The ground men watch the ground, and set the depth of the machine to job specs. They also keep an eye on the cut, to make sure it stays at depth, and keep a smooth operation going.
PM-565 Operator has ability to do almost everything, except raise and lower slabbing plate/lower conveyor. (switch is not on operator platform. Cannot raise sideplates from up top. Operator can do all else, but needs eyes on the ground to keep the sonic sensors from making the machine make bumps in the road or a unsmooth surface.
Groundmen are necessary to keep the milling operation productive. Yes the operator can do it all, but at what time cost, getting on and off the machine to check the cut, stops production, not good for keeping the money flowing.
Most crews in my company have a Forman who also does ground work, possibly operate and may get water. A water truck driver who also does ground work. and the operator, who can do everything if the others are rather lazy. 3 man crew if possible with a lowboy service truck and water truck completely mobile, to get from job to job quickly.