I have known several people who hosted regular casual Wednesday night dinner parties. They were very low-key and generally based around some activity like going to a farmer's market or watching a movie or playing board games. It was generally understood that anyone who was a regular attendee could invite their own guests. So while the parties started with a small group of friends from work, they grew to be a more diverse set of people over time. People would drift in and out and attendance would be different each week.

Although I moved away and don't really keep in touch, I have fond memories of those get-togethers and still consider the people I met at those parties friends. I really like the analogy of a campfire for something like this. All it takes to start a little community is one person to provide a regular meeting place, something to do, and, crucially, some food.