public class Invitee
{
    public string Email { get; private set; }
    public string? FirstName { get; private set; }
    public string? LastName { get; private set; }

    public Invitee(string email, string? firstName, string? lastName)
    {
        Email = Guard.Against.NullOrEmpty(email, nameof(Email));
        FirstName = firstName;
        LastName = lastName;
    }
}