Surprisingly, that actually sort of fits the club I'm a member of - Pontyclun Flyers
Anybody who wants a ride just posts up on the Facebook group when and where they want to start, with a rough idea of distance and speed, and anybody who wants to join just says so in a response to the post.
The person making the initial post generally chooses the route and any café stops.
There are some more or less regular rides, particularly in the summer - Monday and/or Friday mornings, Tuesday and/or Wednesday evenings, and one or two at the weekend.