also being a good builder and conversationalist helps
i literally just sit in my server afk half the time coding or talking to other people and yet people still join. they jet around, talk, look at the fancy builds that are being made, build something small, and later leave.
having non-descriptive server titles + a couple people in the server draws the curious. i never title my servers anything serious - they're always "modding" or "..." or "building stuff" and yet people still decide to join.