I randomly stumbled across a couple of papers about LARPing in Japan a few months ago. This isn't you, is it?
The Power of Just 100 Yen: LARP in Japan
I've heard of underworld although we don't have a local group. Glad to see that the culture has spread overseas.
Belegarth has also had some luck with globalization - in addition to the massive number of US groups there are a handful of Canadian (that's me!) and Mexican groups. There's also single groups that exist (or used to exist) in Italy, Germany, Denmark, China, Australia, Puerto Rico, Hawaii, and probably a few others I'm not aware of. It really only takes one dedicated player to bring an activity to a new community :)
That article was written by a friend, and member of my LARP group, Bjorn Ole Kamm.
Nice to hear about your group. Its best to have a group who works on rule books, and the mechanics of the game. I like to play, and don't want to get bogged down in the rules.