The Staircase in the Woods by Chuck Wendig

Five high school friends, bonded by an oath to protect each other no matter what.

On a camping trip in the middle of the forest, they find something extraordinary: a mysterious staircase to nowhereOne friend walks up – but never comes back down.

Now twenty years later, the staircase has reappeared, and the friends return to find the lost boy – and what lies beyond the staircase in the woods…