I am outside Tokyo now, but I did join the kyousanto while still there and I recommend it. Even if you can't vote you can participate and organize, they are very welcoming. You don't have to worry about some kind of associated red scare getting your visa in trouble, they are a mainstream political party with a large amount of public support. There was a red purge thanks to the US when the party was at its most revolutionary, but that was quite a while ago. It's actually the largest communist party in a non-communist country.

Temper your expectations to some degree as the party leadership as a whole leans more socdem, they tried running to the right as a way of drumming up votes (it didn't work really) and have even distanced themselved from other AES (they have become very anti-China sadly). They replaced a lot of references to ML ideology with "scientific socialism" and advocate non-revolutionary communism. However, there are people in local chapters with a much more revolutionary mindset and who are often the people coordinating events.

I am not yet a citizen but am in the process, renouncing US passport is obnoxious. Will be able to vote afterwards.

One of their biggest causes in recent years is ending the US-Japanese alliance and removing US military bases from the country. Both of which are things that I support and generally receive a lot of widespread popularity.