public void AddUser_Adds1FakeUsers_CollectionHas2Users()
{
//arrange
IUser fakeUser1 = Substitute.For <IUser>();
fakeUser1.Username = "******";
//act
_uut.AddUser(fakeUser1.Username);
var result = _uut.Usernames;
//assert
Assert.That(result.Count, Is.EqualTo(2));
}