I just registered for a Steam account and bought the Magicka Four Pack, which I intend to play with three friends next weekend. How do I send them their pieces of the game?
Those extra games will end up in your Steam Inventory.
To access it, open the Community window in Steam, then click on Inventory in the right column. Select the item you wish to send, and press Send Gift.
It is also possible to directly manage your gifts by clicking on the Games menu in steam and selecting "Manage Gifts and Guest passes".