You could always organize futsal games and the like in one of Tokyo's many parks, but the logistics of getting to parks or the riverside can be somewhat difficult. The solution of course is to seek futsal courts within the downtown areas of Tokyo which everyone can access easily. Not to mention having the courts ready for you to use with balls and nets is very convenient.
There are many small futsal courts here and there, but one good option that even has English services available is the YMCA international wellness center in Tokyo. Here they have futsal as well as other sports for you to play, and the price is generally very good.