Hi Kevin,
just go to "Friends" tab, "View Friends List", switch tab from "Friends" to "Groups" and here you can find and chat with Magicka Rookies members, just ask them for a game. I am sure someone will play. You can also join official Magicka group made by Steam to find even more players.

Just bought the Humble Bundle and looking to co-op Magicka as well as any of the others in the bundle