NOTE: This offer is no longer available, Second giftable copies were only provided during the open beta period.
When you buy into the Hot Lava open beta, the first copy of the game goes straight to your Steam game library (as it normally does) and a giftable copy of the game goes to your Steam inventory that you can send to a friend.
You can get to your Steam inventory by hovering your mouse over your name at the top of Steam and choosing "Inventory". It will be in the Steam tab.