The act of making a first post or comment is all that's required for other instances to become aware of yours. Your site is already in their /instances page and at places like https://lemmy.fediverse.observer/redlemmy.com

From what I can tell re join-lemmy, the list there is just for that site, and it's fed by the lemmy-stats-crawler (you'll be able to see it in your nginx logs). There's a restriction of needing 5+ active users a month though, which it doesn't look like your site quite meets.