The Watch
(2012)

One of the members of the neighborhood watch is actually an alien named Jamarcus (Richard Ayoade), who lets the other members of the watch know that unless they stop the aliens that night, an invasion will occur and life on Earth will be destroyed.

The neighborhood watch kills the aliens (who were camped out at Costco) and avert the invasion.

Jamarcus fights the aliens and remains on Earth, and the neighborhood watch continues to defend the neighborhood.

Thanks Melissa!