title: Unable to add domain users to Sql Server login
body: <p>I have configured SQL Server 2008 R2 for mixed mode authentication on a Windows Server 2008 R2 box. I have logged in using domain administrator account into Windows as well as into Sql Server. Now I am trying to add another domain user account as a new login but I am getting the error saying "Windows NT user or group '\' not found. Check the name again. (Microsoft SQL Server, Error: 15401)" though this user is present in domain. I am adding using Management Studio -> Logins -> New Login. When I search for the domain user using the search button the system searches successfully for the user but when I click ok button to create the login I get the above error. I have tried using T-SQL also to create the login but with the same error.</p>
  
  <p>Need help!!</p>