Box around notes for individual instruments inside a Staff group

[Juan Toro]

Hi everyone!

Currently I'm trying to use the "Box around notes" code that can be 
found in the wiki: https://wiki.lilypond.community/wiki/Box_around_notes

So far, it has function great when outputting instrument parts, but when 
I join them, each box is being drawn for the entire group (see image 
below where boxes should be surrounding the black-head groups). As the 
code is really obscure for me, I don't really see if there is an error 
on me while notating the boxes, or if it's the code that takes into 
account everything that is happening inside the staff group. Could 
anyone tell me if grouping is done by default? If it is, what should I 
need to alter in order to make the box adjust per instrument?

Thank you,

Juan