Asp.Net Identity save user without email

I know this is old, but I disagree with the accepted answer, since the question is tagged as asp.net-identity-2. For the benefit of future readers, ASP.NET Identity 2.0 has a very simple solution to this problem:

public class ApplicationUserManager : UserManager<ApplicationUser>
{
    ...snip...
    public static ApplicationUserManager Create(IdentityFactoryOptions<ApplicationUserManager> options, IOwinContext context) 
    {
        var manager = new ApplicationUserManager(new UserStore<ApplicationUser>(context.Get<ApplicationDbContext>()));
        manager.UserValidator = new UserValidator<ApplicationUser>(manager)
        {
            // This disables the validation check on email addresses
            RequireUniqueEmail = false
        };
        ...snip...
    }
}

In UserValidator<TUser>, the Task<IdentityResult> ValidateAsync(T item) implementation checks this flag and determines if it should run email validation:

if (this.RequireUniqueEmail)
{
    await this.ValidateEmail(item, list);
}

Since you want to save users without an email address, this is how you should do it.

CAUTION: This should only be used when email addresses are not collected. If you want to collect and validate email addresses, but make them optional during registration, you should use a custom IIdentityValidator.

ASP Identity 2.2 can be set in App_Start\IdentityConfig.cs

    manager.UserValidator = new UserValidator<ApplicationUser>(manager)
    {
        AllowOnlyAlphanumericUserNames = false,
        RequireUniqueEmail = false
    };