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.
Website, social media (including meetup or facebook), flyers,
In many jurisdiction, it’s not that complicated to create a small non profit club to collect money to rent a space which is often sufficient to attract people