Adding new groups

You need to register the group via PHP API. This will add a new group both for the "NewContentElementWizard" and the record type selector box. It works the same for Page Types and Record Types. Adjust the table and typeField accordingly.

<?php

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTcaSelectItemGroup(
    'tt_content', // table
    'CType', // typeField
    'my_group', // group
    'My group label or LLL:EXT reference', // label
    'before:default', // position
);