Adding selection to custom group

[CheesyCode]

Hi everybody I am working on a class  project but I seem to be stuck on one thing. How can I add "non-referenced" joints (maintaining their hierarchy) to a new group thats named "UpperBody"? This has to be done in Python, and this will apply for multiple files, so it should work for any file. When I say "non-reference" I mean ignoring joints that were referenced into the file.