The Six by K.B. Hoyle

Darcy Pennington hates her life. She is an insufferably average teenager with no true friends, crushing social anxiety, and an indescribable sense of not fitting in anywhere. When her parents force her to attend Cedar Cove Family Camp the summer before her eighth-grade year, Darcy once again finds herself on the outside of an established social circle. But the camp holds secrets, and when Darcy begins to have strange experiences, she comes to believe she’s either losing her mind or on the brink of a discovery that could give her life purpose. An unwitting tumble through a magical gateway lands her in a new world called Alitheia, and Darcy must convince five other teenagers at the camp to not only befriend her but follow her on a journey beyond their world and their wildest dreams to save Alitheia from an ancient, shadowy foe.