The Tokyo Art Beat website lists most of the art events in Tokyo. If you really want to meet up with artists and people interested in art in Tokyo, your best bet might be to simply look up some upcoming art openings at galleries you find interesting. Show up at the openings and introduce yourself to a few people. Once you meet a few people, you will meet more and more.
Tomio Koyama has an opening coming up in a couple of weeks, but there are others at smaller galleries every week in Tokyo.
Good luck!