Everyone talks about "CLUG" - the Canterbury Linux Users Group. But there is no formal organisation bearing that name.
CLUG exists primarily as a mailing list, run from the University of Canterbury. That's it. Yes, there are occasional meetings run by and for the people on the mailing list, but no formal membership requirements.
The mailing list has an open registration policy - as long as your email address works properly, you can join. When meetings are held, they are open too - if you're interested, come along and meet fellow Linux people. Depending on venue, we sometimes ask for gold coin donations to cover hire costs.
And that's basically it. It works.
"There Is No CLUG" history: http://lists.ethernal.org/oldarchives/cantlug-0301/msg00416.html (30 Jan 2003)
CLUG (the name) history: http://lists.ethernal.org/oldarchives/cantlug-0406/msg00720.html (29 Jun 2004)