As per the Get a User API documentation, it accepts only 2 query parameters namely orgId and userId. However, it is not clear which query parameter is not supported by the API. Are you expecting it to also support groupId as well? If yes, then that would be a feature request. You can submit a feature request with your detailed use case via https://ciscocollabcustomer.ideas.aha.io if you'd like.
Best regards,
Rudrakshi Srivastava