Hi,
I was wondering what is the quickest easiest way to adjust the rear spring for passengers
The only way possible, get up under there with the spanner and skin your knuckles.
search for "rear preload"... you'll get an eyeful
If you are going to carry a passenger just occasionally it may not be worth the bother but if you are going to do it regularly or for a long trip you may want to also ease your chain tension, check/adjust tyre preasure, check headlamp beam position and remember a passenger will affect breaking and accelerating, try to think ahead even more than normal and ride smoother.