Start offline - establish a location, time/schedule, snacks, etc. Advertise online (e.g. craigslist, local forums, via social media such as with school, city, library groups), put up flyers in advance (e.g. weekly starting MM/DD/YY. Try to find a group of core members because someone will need to keep track of things
Exist near the intersection of a fault line and the path of major hurricanes deviated by just the right amount and strengthened by global warming