Create a user from code behind

Hi,
I need to create an asp.net user by using C# code only i.e. at the runtime. The purpose is that i want to allow twitter, fb, ..  based sign-ins in my website. Once they sign in thru twitter say, then i need to create the user in asp.net database automatically if it has not been created. So i need to create an asp.net internal user from code behind only. Kindly help !!! :)
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