Bokurano: Ours

One summer, fifteen kids innocently wander into a nearby seaside cave. There they meet a strange man who invites them to play an exciting new video game. This game, he explains, pits a lone giant robot against a horde of alien invaders. To play the game all they have to do is sign a simple contract. The game stops being fun when the kids find out the true purpose of their pact.

(Source: Viz Media)

The extra story Bokurano: Mou Hitotsu no Bokurano (Yet Another Bokurano) ran after the completion of the main story.