Adrenaline Rush — Cindy M. Hogan

A madman with a mission is kidnapping groups of thrill-seeking high school seniors across the country, and it s up to Christy to stop him.
To do so, she must take on a fearless alter ego and infiltrate a group of adrenaline junkies bent on pushing life to the limit. Death-defying stunts are only the beginning: two groups fit the profile, and Christy must discover the real target before it s too late.
If she chooses the wrong group, more people will disappear. But choosing right puts her as the prime target with no guarantee that she ll get out alive.