A Snow Ninja is a penguin who has completed Card-Jitsu Snow and has received the full suit. To become one, you most likely will have to defeat Sensei in the game and receive the Snow Gem. You cannot become one yet, but you will be able to at the end of February 2013, as confirmed by Spike Hike and Polo Field on Twitter. A Club Penguin tester confirmed that Card-Jitsu Snow will be for everyone, not just Members, even after the beta test, so non-members can be Snow Ninjas.