Co-Op can be played in any of the game's modes. Players work together to fight off zombies. There's a revive mechanic as well.
More Info @ PCGamingWiki
Nation Red is an ultra fast infinite play arena-based shooter providing an exceptional close-quarter battlefield experience. Fight thousands of zombie mutants and their bosses in frantic bloody gameplay while earning over 70 perks.
August 17, 1977
Scattered groups of zombies infiltrate remote Midwestern towns. Incapable of using tools or weapons, the invaders are resisted until they retreat back into the desert. Special elimination posses are set up to hunt down any of the surviving undead. Six weeks after the squads started their search-and-destroy mission, the last known zombie was shot and killed.
Over thirty years had passed when a truck driver traveling down a desolate Arizona road plows into a wall of undead. Before the driver's phone signal went dead, he reported some were carrying guns... They're back and now they are hunting us.