At some point, in order for change to actually occur, things would have to move "off-line" and people would be forced to meet in public, wouldn't they?
Assuming the group had been infiltrated (or monitored), it seems like that might very well be the end of the group.
I think that in the NK scenario, meeting offline would be one of the last things that would happen. Just being able to anonymously communicate with each other would be pretty revolutionary, as so much of the way the society functions is based on lies that unravel once people can access/share/discuss outside information.
It seems like once there's enough popular support, it'd be a lot easier - there are a lot more of the populace than the ruling class?
Cory Doctorow actually writes the most plausible scenario (and pitfalls) of bootstrapped online/offline coordination that I've read in Little Brother... based on how NK works, infiltration of any group, infiltration is probably a given. The real question would be whether the gov't could respond if the majority of the population were all conspiring in groups. Are there enough secret police to execute/arrest everyone all at once?
Assuming the group had been infiltrated (or monitored), it seems like that might very well be the end of the group.