With an account on md.bassdrum.org you'll be able to follow people on any Mastodon server and beyond.

BASSDRUM - mastodon への参加は招待制です。BASSDRUMの管理者から招待リンクを受け取り、そちらから参加してください。

You can join in BASSDRUM - mastodon only with invitation. Please get invitation from BASSDRUM admin and join with that.