Hi. Have you emailed the Tokyo chapter? We don't seem to be as active as one would expect but could probably point you in the direction of some study groups.FYI, A text to the Twitter or LinkedIn page usually gets a reply faster than email.
I know you said that you would take the exam in English or Chinese but do you understand Japanese? If so there is a free CISSP "Challenge" Seminar on October 8th.
You can also check out the Asia Pacific Chapter Groups here on the Community. They may have information to share:
The Japan Chapter at the link you provided is apparently not very active if at all. Only 8 members and I made a request to join in September and still is "pending".