title: Add an external user in a group
body: <p>Using <code>System.DirectoryServices.Protocols</code> (S.DS.P), I need to add <em>UserX</em> from directory A to <em>GroupY</em> in directory B.</p>
  
  <p>My <code>ModifyRequest</code> contains DistinguishedName of <em>GroupY</em> and <em>UserX</em>.</p>
  
  <p>Send request throw a <code>DirectoryOperationException</code> with result code <code>NoSuchObject</code>.</p>
  
  <p>I see, however, that the same operation is workable using the ActiveDirectory console (MMC snap-in). A <strong>ForeignSecurityPrincipal</strong> object is created in directory B and referenced as member of <em>GroupY</em>.</p>