You can use the same ideas for reading groups for teenagers as you use for adults.
The following questions are used to review books for ALA | Best Books for Young Adults and they might help you with getting discussion started.
How well did the author capture your attention?
How vivid were the details?
How well did the book build in intensity?
What did you think of the ending?
Did you find the characters believable? How did you feel about them?
How much did you enjoy reading this book?
How highly would you suggest this book?
How well did this book match your morals and ethics?
How memorable was this book?
How 3-D and alive was this book?