UserManager.Default.CreateUser(userName, password, email); means that you will use the Default Membership provider explicitly. There should not be a problem if you use this code
var manager = new UserManager();
manager.CreateUser("name1", "mypasword", "firstname.lastname@example.org");
You can get null from CreateUser if something fails internally in the method. For instance you have set requiresQuestionAndAnswer="true"
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