This is what I was taught.
Preferably with everything stopped. This is the only way my wife will do it.
If it must be rotors running, do it with RRPM at flying speed, pax approaches in line of sight of pilot - not on a slope, and at a pre arranged signal from pilot. Pax keeps head down, pilot keeps tip plane up. Keep eye contact. Pax gets in. Do not allow boarding at idle as the tip plane will be much lower and the pilot cannot control it. Getting them out is pretty much the reverse. It may be different for other types.
Blade sail? With an Enstrom once the grips are on the droop stops the pilot has no control over the rotor. You can do what you like with the cyclic it makes little difference. It takes a big gust to upset an Enstrom rotor.
With a teetering head one does ones best with the cyclic and its in the lap of the gods.