Note: This game already is free on Steam. However, the devs pulled some stuff behind the scenes to allow people to get trading cards by "redeeming" a key for the game. All you have to do is join the Steam group (and leave afterwards, if you want).
This key will allow you to get Steam Trading Cards from Relic Hunters Zero. If you already own/installed it, go here: https://store.steampowered.com/account/licenses/ and click "Remove" next to Relic Hunters Zero, then try the key again.