A simple one.
Run a database query, use Mailchimp, or any other tool to highlight members that live in New York, Los Angeles, and Chicago*.
Send an e-mail out to each group individually to ask if anyone would like to be responsible for organising a monthly community meetups for their city. They get your promotion/support and the status of running a meetup. Better, offer $200 to sponsor each event (restaurant/bar/activity).
They get to pick the activity for the group and how they run it. You get a stronger, tighter, group.
Now move on to Houston, Philadelphia, and Phoenix.
*The 3 biggest cities in the USA, pick whichever works best for your audience.