Gantz (2010)

A group of strangers who may or may not be dead are transported to a small room where a large black orb called Gantz provides them with body armor and weapons, and sends them on a mission to kill an alien living on Earth.

The two main characters are teenagers who were best friends when they were children, but now they are essentially strangers.

They are teleported to a location near the alien with instructions to find him and kill him.  They must learn how to fight and use their weapons, or their intended victim may kill them.  Some players don't want to kill anyone. Others are willing to do what they have to as it keeps them alive.

Each person is awarded points based on their battle and the most going to the person who kills the alien.  Once they get 100 points, they can go back to their lives with their memory of this erased, or they can resurrect a dead player.  Since players often only get 1 point in a mission, it is almost impossible to get the required number of points.

Once the mission is over, they are transported back to their regular lives.  But they never know when Gantz may summon them again, and by summon, it means teleport them to the white room for another alien killing mission.

The movie is based on Anime, which I haven't read.  That's probably a good thing as I have nothing to compare it to and can't be disappointed if they take liberties with the story.  I liked this movie. It's bizarre but it's another one where there's a second movie which wraps up the first.  It wasn't as much of a cliffhanger as the Death Note movies, but just keep that in mind.