In this federated world, you need to go to the top most populated instances, make a user account, then subscribe to your community from each instance. This gets your posts into the all feed.

Advertise your community in new communities

Pump the numbers to get into trending communities.

Make a bunch of cross posts, to “accidently” raise awareness.

Tell people about it in comments with. Oh hey, we talk about this stuff in !community@example.ml