private static void AddContactToGroup(EsendexCredentials credentials) { var groupService = new GroupService(credentials); var contactService = new ContactService(credentials); try { var guid = new Guid("{YOUR Contact GUID}"); var contact = contactService.GetContact(guid); groupService.AddContactToGroup(_accountReference, "{YOUR Group GUID}", contact); } catch (WebException ex) { Console.Write(ex.Message); } }