For any range of topics, lighthearted to serious, hobbies and games to helping and engaging with community matters. Regarding reach, I mean what ways may be used alongside word of mouth, as I honestly don’t hear people talk about groups they’re in (besides sports/bands) all that much.
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
never underestimate the power of punch and pie