I used a high key backdrop at AB06 for my staff work.. is this what you mean?
Doing work like this is actually more than just flipping a backdrop out behind the people.
It starts out as Ollie states. Whether you use a muslin backdrop or paper is kind of irrelevant.
In the images in that gallery of mine, I used muslin. I chose muslin because it is far mroe transportable. Since I was doing staff work at a convention, I had to transport a studio to the convention.
You need a fairly LARGE backdrop to start with. Once up you need it to be completely free of wrinkles. Both in the backdrop, and the floor underneath the cosplayers. This means you spend a significant amount of time getting wrinkles out. Probably for every minute shooting you have 3 removing wrinkles and folds in the muslin. I also bring a professional steamer with me to aid in keeping the backdrop wrinkle free.
The reason the backdrop needs to be wrinkle free, is because lights that cast across the backdrop wil create VERY obvious shadows over any wrinkles and will ruin the effect.
NEXT, you need at the VERY LEAST, one backdrop light to overexpose the backdrop. Set this light to 2 stops over your key light's reading on your subject.
Your subjects need to also be moved forward from the backdrop, 6-8 feet MINIMUM. This removes any potential shadows from your main light, as well as any potential light spill from your backdrop light(s).
Your main light needs to be diffused properly, and you may choose to use a reflector or a fill light to grab the shadows on the off side of the cosplayer.
That backdrop was 24ftx10ft. I would say it had about 10 feet up, and 14 feet on the ground. It accomidated 4 cosplayers comfortably. Anything over that was beginning to crowd the backdrop.
Set your camera to your meter your main light.
Ensure you have PLENTY of room for this sort of thing. And expect to photoshop areas of wrinkles that you missed. The more work you do before you snap the shutter, the less work you have to do after you're done. So be a stickler for details.
Feel free to ask any questions.