The spring spanner adjuster sets the Sag of the bike, or the height of the rear suspension, measured from some point of choice to the ground with weight off the shocks, then taking the difference of the same measurement with the weight of the bike ON the shocks. This should be pretty close to right, out of the box, but may require some fine tuning. Make note of this height of the rear suspension somewhere, as this is where you want the bike to sit regardless of load. So you'll probably end up with a setting on the spanner for solo, solo w/load, two up w/load, depending on how you ride. The spring preload only sets the height of the bike, and should not be mistaken for "stiffness".
The rebound collar, located near the "knuckle" of the shock, with settings of 1-5, should be set for controlled rebound after bouncing on the bike. I have found that 3 works well for most cases on the Vrod. This setting keeps the shock from "topping out", or becoming completely unsprung.
The compression damping knob, with settings from "H" to "S" (hard to soft) effects the compression ability of the shock, or how quickly the spring will compress. This adjustment is like a tapered needle valve...the value increases exponentially with movement of the knob towards the "H" setting. In other words, on 6 clicks away from "S", movement 2-4 more clicks towards "H" increases the compression damping ability of the reservoir more and more with each click...it is not a linear relationship from "S" to "H". Each click does not produce the same relative result. As Chief said, start on a middle position, and increase as needed to keep the shock from bottoming out.