public class ApplicationUser : IdentityUser
{
    public async Task<ClaimsIdentity> GenerateUserIdentityAsync(UserManager<ApplicationUser> manager)
    {
        // Note the authenticationType must match the one defined in CookieAuthenticationOptions.AuthenticationType
        var userIdentity = await manager.CreateIdentityAsync(this, DefaultAuthenticationTypes.ApplicationCookie);
        // Add custom user claims here
        return userIdentity;
    }

    public Customer Customer { get; set; }
    [Required]
    public int CustomerId { get; set; }
}

public async System.Threading.Tasks.Task<bool> AddAccount(string email, int customerId)
{
    // Asp.net User
    var user = await UserManager.FindByEmailAsync(email);
    string password = Membership.GeneratePassword(5, 1);
    if (user == null)
    {
        user = new ApplicationUser { UserName = email, Email = email, CustomerId = customerId };
        var result = await UserManager.CreateAsync(user, password);
        if (!result.Succeeded)
        {
            return false;
        }
    }

    return true;
}