I haven't joined ROK Riders yet, (I don't think) but I will do that soon.
I already loaded Open Street Maps on my Zumo, and it seems to work pretty well so far. It knew the best route to get through the city to my apartment, although it took me about a block out of the way one time.
I'd be up for a ride to Gwanju, and was actually thinking of trying to ride out there anyway. It would be much better to be with a group though.