If one of the desired_members does not exist, then the `name_to_sid_object` lookup returns nil, which later leads to `undefined method `account' for nil:NilClass`. Previously, this wasn't an issue because everything was just a name, and the error would be raised by ADSI. We should probably raise an error when trying to add or remove a non-existent user to a group, or a non-existent group to a user.