Ok, got it. The proble was at GetObject() Function needs group name to be deleted and the OU of the group container.

 Code:
Function DeleteADGroup($StrGroup, $strGroupOU)
  Dim $objOU
  ;
  $DeleteADGroup = 0
  $objOU = GetObject("LDAP://" + $strGroupOU)
  If @ERROR
    Exit
  EndIf
  $objOU.Delete("Group", "cn=" + $StrGroup)
  If @ERROR = 0 
    $DeleteADGroup = 1
  EndIf
  $objOU = ""
EndFunction