Bokurano(2007)

During a summer camp, 15 children, 8 males and 7 females, find a grotto by the sea. Deep within they discover working computers and some electronic equipment, and later the owner, a man called Kokopelli. Kokopelli claimed to be a programmer working on a brand new game, in which a large robot has to defend the Earth against alien invasions. He persuades the children to test the game and sign a contract. All but one of them signs, barely a moment later they mysteriously awaken on the shore believing what happened was just a dream.

(Source: Anime News Network)