Tried with the below code, it's throwing an error.
MetadataService.MetadataPort service = createService();
MetadataService.Group_x CreateGroup = new MetadataService.Group_x();
CreateGroup.fullName = 'Test';
CreateGroup.doesIncludeBosses = false;
CreateGroup.name = 'Test';
List < MetadataService.SaveResult > saveResultList = new list < MetadataService.SaveResult > ();
saveResultList = service.createMetadata(new MetadataService.Metadata[] {CreateGroup});
Error: Line: 13404, Column: 1 System.CalloutException: Web service callout failed: WebService returned a SOAP Fault: Unable to determine type mapping for type {http://soap.sforce.com/2006/04/metadata}Group_x. Type is illegal here. faultcode=soapenv:Client faultactor=