I belong to a tennis Meetup that does this and like the ability to pick up a random match. The way it works is you RSVP YES for this non-event and make sure you are subscribed to comment alerts. (It is the default setting so if you have not muted notifications you are subscribed)
When someone leaves a comment saying that they want to ride at a specific time/place etc. you will receive a comment notification. If you want to join them reply to the comment and work out the details.
When suggesting a ride please begin with the date and time of the proposed ride. Follow that with your proposal. If the thread becomes long and complicated it may become necessary to begin a new thread with the set details so that new comers to the thread can easily determine those details.
The excellent CFH event organizers try and maintain a friendly, no drop, culture for this cycling Meetup. These “impromptu” rides are not organized events by CFH event organizers and shouldn’t be confused with actual CFH events.