public void AddGroup(int ContactId, int GroupId)
 {
     try
     {
         ContactService.AddGroupToContact(ContactId, GroupId);
     }
     catch (Exception)
     {
         throw new HttpResponseException(new HttpResponseMessage(HttpStatusCode.InternalServerError)
         {
             Content      = new StringContent("An error occurred, please try again or contact the administrator."),
             ReasonPhrase = "Critical Exception"
         });
     }
 }