Broctoon refered to the key elements: same time, same place, and even the same route(s) every week.
I think that’s the way the go, and would even give it a catchy name, like Taco Tuesday Ride, and loop in a local business that’s the social focus and start/end point.
Put the route on RideWithGPS and make a Facebook page with the event name, and you’ll be off and running. You’ll need to advertise, promote and recruit until you get a solid core of regulars, but after that, it can go autopilot. You’re gonna have to pick a target audience, though, if you don’t want to do the heavy lifting of organizing, and cater to that element. Is a fast ride, or slow…recovery or training or sportive? Just be clear in your promo and let people know; be sure to mention it’s a drop/no sag ride, too, but with the route on RWGPS and other social media formats like Strava, that should not be a problem, and groups will form on their own.
I do think the post-ride social aspect is critical for it to be a ride with a life of its own, though, so the hard work will be in finding the partner venue that appeals to, and is willing to embrace, cyclists. Around here, it’s businesses like breweries and coffee shops primarily, but just think about the kinds of things which appeal to cyclists before you go out looking for the ideal spot: you’ll want parking, outside seating with space to keep an eye on bikes, more than one bathroom, etc. Of course, it also has to be a place where it wouldn’t be weird to waddle around in cleats and lycra, too, so keep all that kind of stuff in mind, and “set the table for longterm success.”
Good luck,
BrenZan