Make sure you've got a Meetup scheduled. It gives potential members a very compelling reason to join, and gives current members something cool to look forward to!

Write a good Group description.Your Group description tells the world what your Meetup Group is all about, and what people can expect if they join.

Encourage your current members to fill out their profiles. For people thinking about joining a Group, one of the most important things to find out is what sorts of people are already in this Group. You can set your Group up so that new members have to have photos or answer particular questions. But even if you decide not to make the questions mandatory, encouraging your current members to fill in their details will help the Group find new members in the future.

And finally, don't forget to have fun! People gravitate towards Groups with happy Organizers and satisfied members.