If the bullies are already on a transparent background, then you just overlay their layer on top of the layer with the girl.
Compositing is rarely that simple, though; the devil is in the details. When you need help with those details, be sure to post screenshots so that we can see exactly what you're seeing.
Start here to learn After Effects:
http://adobe.ly/bjBT3P
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Todd Kopriva, Adobe Systems Incorporated
After Effects quality engineering
After Effects team blog
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