This is based on how I imagine the characters to be and on my little knowledge on modern day actors. They are in order of how the appear in the book. Well, the teens are introduced in order and then the rest are just randomly placed.
Also, I don't put Michael's and John's last names because I didn't actually ever give them one so...yeah.

Hailee Steinfeld as Emma Reynolds

(I know I describe Emma with blue eyes so lets pretend Hailee has blue eyes)

Shawn Mendes as Shawn Becker

Meghan Ory as Tamara Winchester

Grant Gustin as Vincent Mathews

Katherine Mcnamara as Ashley Adams

Victoria Justice as Kasey Rivers

AnnaSophia Robb as Michelle Rivers

(The twins look nothing alike, I'm aware of that but in the book they're described as very different)

Keegan Allen as Michael

Brant Daugherty as John

Blake Jenner as Logan Reynolds

Julie Bowen as Karen Reynolds