There are a variety of ways to grow your numbers. My approach typically yields 100-150 ppl per month with numbers ballooning to 300-500 when I "step it up."
The secret to my consistent larger numbers each month comes down to one thing - my monthly email marketing sequence.
Sure there are other things you can do.
Use different platforms to attract ppl - like Meetup etc.
You can implement a "Bring a Buddy" initiative - this doubles your numbers quickly if they do.
You can implement door prizes, giveaways and other interactive contests that you promote in advance to draw larger numbers.
But in the end ... it all comes down to how you market and message each month. I can't easily explain the monthly cycle in this answer box, but I'm happy to shoot you a free copy of my best selling book MEETUPOLOGY that explains much of what I did (and still do) to make the numbers of which you speak.
Let me know. And naturally I'd be honoured to speak with you directly too.
Writer & Digital Coach