How do offline, local groups get started and reach others now?
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.
Some cities have gaming stores with active groups and tournaments and learn-to-play events. Public libraries often have a calendar of community events. Restaurants sometimes have them, too. Community theater. Bridge clubs. Churches often have secular events like concerts, games, fairs, book clubs, etc. Maker spaces. Community college adult education. Hobby classes like woodworking and pottery.