code: void openAddUserDialog() {
  
          CmsPrincipalSelectDialog dialog;
  
          final Window window = CmsBasicDialog.prepareWindow(DialogWidth.max);
  
          dialog = new CmsPrincipalSelectDialog(
              this,
              m_stateBean.getPath(),
              window,
              WidgetType.userwidget,
              true,
              WidgetType.userwidget);
  
          try {
              dialog.setOuComboBoxEnabled(
                  !OpenCms.getOrgUnitManager().readOrganizationalUnit(m_cms, m_stateBean.getPath()).hasFlagWebuser()
                      | m_stateBean.getType().equals(CmsOuTreeType.ROLE));
          } catch (CmsException e) {
              LOG.error("Can not read OU.", e);
          }
          window.setCaption(CmsVaadinUtils.getMessageText(Messages.GUI_USERMANAGEMENT_ADD_USER_TO_GROUP_0));
          window.setContent(dialog);
          A_CmsUI.get().addWindow(window);
  
      }
docs: opens a principle select dialog.<p>