A necessary (but not sufficient) condition is that the people who want to attend meetups can find out when and where they are happening. You might think this is obvious, but my local group requires a login in order to view its "public calendar".

I'm not sure I fully understand what you're asking; are you asking:

1. How do you get group members/EAs to physically show up to meetups?
2. How do you get group members/EAs to show up either physically or virtually to meetups?
3. How do you get people in general (including people who may not be very familiar with EA or already part of your group) to become sufficiently interested/etc. in your local EA group that they show up to meetings?
4. (Something else?)

These resources on promoting your group and events from the EA Groups Resource Centre might help! https://resources.eagroups.org/running-a-group/promotion-advertising

Very old guide, maybe a tiny bit helpful: How to Run a Successful Less Wrong Meetup Group.