[TOMT][Book] Weird book that involves a group of kids who try to save their friend but have to give up 1 year of their lives to do it

I read the book about 10 years ago, give or take, but I have absolutely no idea when it was published prior to that. I don't remember that much about it, other than the fact that it was a group of kids (maybe teenagers?) whose friend gets kidnapped or lost or something, maybe in a place similar to hell or some sort of underworld (not too sure about that part). They eventually find him, but the antagonist won't let them have him back unless the group unanimously agrees to die one year younger than they would have. I think one of the examples was "if you were supposed to die at 45, you'll die at 44" or something like that. It scared the fuck out of me, but I'd really like to revisit it. Does anyone have any idea what this is?