Four player co-op is available in all game modes and types including a story mode and an unlockable survival mode. While you can't drop in to a game, you can drop out and not break the session if you aren't the host.
More Info @ PCGamingWiki
Magicka is an action-adventure game set in a rich fantasy world based on Norse mythology. The player assumes the role of a wizard from a sacred order tasked with the ultimate goal of stopping an evil sorcerer who has thrown the world into turmoil, his evil creations besieging the forces of good.
Players will be able to combine the elements to cast spells, wreaking havoc and devastation on the minions of darkness. They will also be able to team up with friends and fight their way through the campaign, or test their skills in the magickal arts in other challenging modes.
This page contains co-op info for Magicka on the PC. All information about Magicka was correct at the time of posting. Information is subject to change. If you see any errors please email us.