Nintendo has opened up registration for its new account system, as well as letting users sign up for its new iOS and Android app Miitomo.
You can register for a Nintendo Account using your Nintendo Network ID, Twitter, Facebook or even Google+ account, and you’ll want to be fast to ensure you get the username you want – or the closest approximation.
The Nintendo Account can be used for Nintendo’s upcoming rewards program as well as Miitomo, the company’s first smartphone/tablet app. Right now, though, you won’t be able to do much with it.
Creating a Nintendo Account will also let you pre-register for Miitomo, which will arrive next Month. Although there’s a lot we still don’t know about it, Miitomo will be somewhere between a game and a social network, in which you’ll interact with friends via your Mii avatar.
Source: techradar.com
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it just shows an error when you try to register..
You use your email address to login, and your display name can be whatever you want (regardless of whether anybody else uses the same). So there's no chance of being stuck with a username you don't want.