// make those fields public for accessibility
public string Username;
public string Password;
public string NAME;
public string Age;

public User(string Username, string Password, string NAME, string Age)
{

    this.Username = Username;
    this.Password = Password;
    this.NAME = NAME;
    this.Age = Age;
}

public static void AddUserToList(User user)
{
    UserList.Add(user);
}

{


    if (string.IsNullOrWhiteSpace(NTB.Text) && string.IsNullOrWhiteSpace(ATB.Text) && string.IsNullOrWhiteSpace(RegUserNameTB.Text) && string.IsNullOrWhiteSpace(RegPasswordTB.Text))
    {
        MessageBox.Show("Please enter the following:" + "n" + "Name" + "n" + "Age" + "n" + "n" + "UserName" + "n" + "Password");
    }
    else
    {
        User user = new User(RegUserNameTB.Text, RegPasswordTB.Text, NTB.Text, ATB.Text);
        User.AddUserToList(user);
    }
    Close();
}