MCV3 Entity Framework 4 Code First: Adding Sub Objects to the db
public class User
{
    [Key]
    public int Id { get; set; }
    public string User_Name { get; set; }
    public string Password { get; set; }
    public string First_Name { get; set; }
    public string Last_Name { get; set; }

    public virtual ICollection<Email> Emails { get; set; }
}

public class Email
{
    [Key]
    public int Id { get; set; }
    public string Address { get; set; }

    public User User { get; set; }
}

public class dbInitializer : DropCreateDatabaseAlways<UserContext>
{
    protected override void Seed(UserContext context)
    {
        var Users = new List<User>
        {
            new User {  User_Name = "uname",
                        Password = "pword",
                        First_Name = "fname",
                        Last_Name = "sname",
            }
        };

        Users.ForEach(u => context.Users.Add(u));
    }
}

protected override void Seed(UserContext context)
{
    var Users = new List<User>
    {
        new User {  User_Name = "uname",
                    Password = "pword",
                    First_Name = "fname",
                    Last_Name = "sname",
                    Emails = new[]
                    {
                        new Email { Address = "email1@domain.tld" },
                        new Email { Address = "email2@domain.tld" },
                    }
        }
    };

    Users.ForEach(u => context.Users.Add(u));
}

var user = Users.First();
user.Emails.Add(new Email { Address = "email3@domain.tld" });

user.Emails.ToList().Add(new Email { Address = "email3@domain.tld" });

protected override void Seed(UserContext context)
{
    var Users = new List<User>
    {
        new User {  User_Name = "uname",
                    Password = "pword",
                    First_Name = "fname",
                    Last_Name = "sname",
                    Emails = new List<Email> { new Email { Addess = "foo@bar.baz" } }
        }
    };

    Users.ForEach(u => context.Users.Add(u));
}

user.Emails.Add(new Email { Addess = "foo@bar.baz" });

var email = new Email { Addess = "foo@bar.baz", User = user };
context.Emails.Add(email);