Hi Diesel,
It seems on a cursory view that Ozstoc members would need to join individually. Once a location is loaded onto the "Geocaching" website it is open for all to use. However, we could mark them like "Ozstoc - Sydney", Ozstoc - Queensland" etc for members (including public so to speak) to find easier when using the search function.
Also, Geocaching is worldwide, so if you want something to be sent to ST riders somewhere else in the world, you attach a "travel bug" which is like an army dog tag to the object you wish to send and post it's location on the Geocaching website, and then just monitor, as it moves around the world.
You will find lots of cache's around airports awaiting their trip somewhere in the world.
It is free to join and it is worth joining and go through the "FAQ" and "Geocaching 101".
Cheers
Dave