GitHub-Style Collaborative iTunes Backup?
Sounds like you want to do the following:


Have your itunes music stored to an external hard drive on download
Have a subset of the music locally on your laptop
Have a centralized repo, where your fellow DJ(s) can access the music


There are a few problems with a simple rsync between your external drive and their drives:


m4a rights managed files won't play on your friend's laptop without you granting their laptop access
as files are played, some meta data on the files are updated to reflect plays, likes, etc.


Having said that, you can work around the issues if you are willing to forego some of the meta information and if you are ok with authenticating your friends' laptop to use your music.


Dedicated Mac Mini or Mac Laptop or Windows machine running iTunes, with the full library downloaded and shared via the normal Apple Airplay/Network share
Your laptop and your DJ friends' laptops can then view/browse/stream the music from the central box. Meta information preserved as well as no need to move a hard drive around, all music available from one central place.
You may need to authenticate your friends itunes to be able to play your rights managed music.


The external hard drive route is basically the same, except that you basically rsync from your mac mini/archive box to an external drive. Requirement to auth a DJ friend's laptop still applies. 

For your laptop which has a subset, you would just choose the songs you want from your library and import them locally into your instance of iTunes.

Re: Github style, probably not the best, since leveraging git would result in massive storage magnification anytime the play counter is updated/etc. Since the files are binary, diffs can't be stored.