Dan,
For all track or closed circuit events a CAMS license is required ( see www.cams.com.au for more info). To get the license you must be a member of a CAMS affiliated club. If the event is not open to other clubs, or only specific clubs, then the club you belong to would be relevant too.
For a cruise, observation run etc. only club members or those from an invited club may participate (afaik, I'll check into this). This may be able to be varied depending on how we write the rules. No special competition license is required in this case.
Anyone can attend a general club meeting or social event.