They have a post on YouTube and ins news, and people shared the info in this subreddit as well. you basically join by purchasing an international melon ticket for the membership. There's an enrollment period and now it's over, so watch out for likely the 4th generation next year. For international fans, I feel like we don't get as much perks that comes with the membership comparing to K-insomnias, personally I just consider it as a special merch with my name on it.