If You Tell Anyone, You're Next by Jack Heath

JAYDEN JONES IS MISSING.

Everyone thinks he ran away. His best friend, Zoe Gale, knows they're wrong.

Zoe's search leads her to The 17 - a secret group chat, used by anonymous teens to blackmail the powerless. To join, you have to put on a mask and record yourself completing a challenge. The challenges are always illegal. 
Sometimes dangerous. Maybe deadly.

Who are The 17? What have they done to Jayden? And what will they do to silence Zoe? 

A rollercoaster of suspense and twists from the author of the Liars series.