Body is an ARRAY of groups. site, group_key and translations (all active languages of the site) are required; id is forbidden (CD0031). Validation is all-or-nothing: if any item is invalid, nothing is created and the response only contains the invalid items ({success: false, group_key, errors}). Otherwise each item returns {success: true, group_key}, {success: false, errors: ["Creation failed for group_key '...'"]}, or {success: false, message} for an unexpected field (CD0079). Business errors are returned with HTTP 200 and success: false; each message is prefixed with its code (CDxxxx - ...).