Because the Brighton Mainline runs through the night - or at least down to Three Bridges, anyway - there are usually regular services to London Victoria, Selhurst / Norwood Junction, and Three Bridges depots. There are also one or two staff trains/buses between Brighton and Three Bridges, and a few via Redhill.
It is generally accepted that stops at either Selhurst or Norwood Junction in the small hours serve to cater for workers throughout the whole of the depot complex and both traincrew depots. These stops are probably the most well-known amongst non-railway-employees, and unless the platforms are unsuitable for public access, most staff stops on trains in public service will see doors released throughout the appropriate coaches. Trains which run with no public stops, as in the link in the post above, are obviously not advertised or accessible to the public. Often, these are effectively run as glorified ECS movements, for simplicity. Under no circumstances should any member of the public attempt to board a train which is marked as "Not in service" (for pedants - unless given permission because of a fault with the information systems).
There are also decent car parks at most Southern train crew depots, although - like walking routes to station platforms - they may involve quite a walk from the actual booking-on points.
As you can tell, I'm not just going to post specifics on here, especially and obviously about depot access. However, special staff stops on public passenger services can quickly be found with discerning searches on Realtime Trains and even (in a hidden sort of way, and in real time) on NRE/popular phone apps.