I am not a professional, but have shot a few events. I would use a minimum of 2 lights at 45 degrees one to each side.Try not to place the lights too high as i am sure you will also be doing some full or 2/3 body pictures so you will need the lights to cover not just thier faces, but their bodies. I do this as with only one light, you would have to be straight on with the light to get proper exposure on more than one person. I play it safe. I would rather have flat even lighting than risk having one person under exposed while the other person is properly exposed.
If you have a hair light, that adds to it. I use my three flash set up with a cheap $10 1/8" grid from amazon for the hair light. These grids are not in degrees, but it is just large enough for two people. The 1/4" grid is designed for one person. Be careful to place the people at the same spot to keep the hair light on their heads and not spilling onto the front of them if you use one. I set this light behind and to the side so that the umbrella stand is not in the picture.
Be careful about the background. Having some props hanging on the back ground may make it look like there are things sticking out of peoples heads.If it were a black background with perhaps a lot of of stars hanging, enough so that you can tell what they are in the pictures, that should work. If you can't get a proper background, i would just suggest using solid black and give the people some props to hold.
I would also like to hear some more input on this to help me improve or perhaps see things that i have over looked.