In Nightfall — Suzanne Young

In the quaint town of Nightfall, Oregon, it isn't the dark you should be afraid of -- it's the girls. The Lost Boys meets Buffy the Vampire Slayer in this propulsive novel from the New York Times bestselling author of The Treatment.

Theo and her brother, Marco, threw the biggest party of the year. And got caught. Their punishment? Leave Arizona to spend the summer with their grandmother in the rainy beachside town of Nightfall, Oregon -- population 846 souls.
 
The small town is cute, when it's not raining, but their grandmother is superstitious and strangely antisocial. Upon their arrival she lays out the one house rule: always be home before dark. But Theo and Marco are determined to make the most of their summer, and on their first day they meet the enigmatic Minnow and her friends. Beautiful and charismatic, the girls have a magnetic pull that Theo and her brother can't resist. 
 
But Minnow and her friends are far from what they appear.
And that one rule? Theo quickly realizes she should have listened to her grandmother. Because after dark, something emerges in Nightfall. And it doesn't plan to let her leave.