1.Sign in to this website
2a.Then go here
or (you can do both though)
Gotham City Impostors PSN closed beta giveaway
3.Click on claim your freebie at the bottom
4. Redeem on xbox 360 or ps3
Another website for more codes.
GameSpot Beta :: Gotham City Impostors
The closed beta begins at 4pm on Thursday 15th December and runs until 20th December.