I started my Historical Fiction Club on 19 March and got to 50 members yesterday, exactly a month later. I'm pretty pleased with that growth for a little niche hobby community. Here's what worked for me:
~ Sending personal invitations. I sent individual DMs (some parts personalized, some parts copy-pasted) on Instagram and Facebook to people I suspected would be interested. This was my biggest source of members.
~ Posting a couple of times each on Facebook and Instagram.
~ Sharing in my email newsletter (and in a couple of related email lists I run).
~ Posting in related Skool groups (Skool was my second biggest source of members).
~ Asking current members to invite friends they thought might be interested.
~ Running a contest to encourage members to invite new people. The prize was a US$50 gift card to a bookstore of the winner's choice, which makes sense for a community of book lovers.
I haven't run ads because this is a hobby community and I can't justify spending that money on it, but if this were a business, I'd for sure try running Meta ads!
What have you tried? What's worked?