I was in exactly your position a couple of months ago - reasonable amateur photographer but not much 'studio' experience (mainly landscape), with a D200, 50 f1.8 and decent flash.
I read a load of tutorials; my favourite being
the strobist, and then bought a light stand, bracket, umbrella and 4ft reflector from
Ebay (should have bought a reflector stand and a backdrop and stand too in hindsight...)
I got a friend to spend a couple of hours patiently posing before the real model turned up and went through the various basic poses and lighting set-ups that I'd read about.
It quickly became very clear that only certain poses were going to work for our purposes (T-shirt shoot) and this saved a lot of time when the real model turned up.
The other thing that came in useful was a big air circulator/fan which gave the model's hair lots of life.
You've got more of a challenge as you have more than one person so may want to consider more than one light source.
Anyway, my results are
here - they're not bad given that I'd only really done landscape photography before...
--Edit--
I used the remote capabilities of the D200/Sigma EF500 which works using a series of pre-flashes; its not as convenient or easy to configure as pocket wizards or the like, but if you've already got a modern Sigma/Nikon flash then its 'free'.