When I lived there ,the Tokyo PC Users group was about the closest thing I found. I don't know how active they are now but it might be a good place to make some connections to other groups. You may also investigate what Ruby groups exist as the creator of that language is Japanese.
Edit: Here is the link to the group's website: http://www.tokyopc.org/index.php
They do seem like they are still quite active since I last attended (over six years ago).
