I'm in a cycle club and they use a webpage with upcoming rides where anyone can publish an upcoming ride with the distance, route and speed target, capped at a certain number of riders. I'm also in offshoot group from the club who use whatsapp to publicize rides but in a very informal basis as the club rides tend to require lots of organization and questions like - "anyone have a 6mm Allen key to raise Barry's seat..."

A few thoughts:
- There may be some value in having an app for existing clubs. Eg they can use a paid-for admin account to maintain membership, fee payments, info page, upcoming events etc and members can publish rides. I'm sure maintaining a web site is a major task for most clubs and replacing this with a 3rd party app would be welcome. Perhaps charging clubs say $99/year and then the club offering paid membership (say $25/year, first 5 rides free) of which you get a cut. The club model may work for other clubs like hiking, horse riding or similar. Or maybe only charge clubs if they have over 100 members to encourage new clubs.
- Make the rides be able to limited to those who have completed previous rides. Eg in order to sign up for a 100mi "A" pace ride, you must have completed a 50mi "A" pace ride and to do the 50mi ride you must have completed a "B" pace ride and so on.
- Liability issues. If an organizer plans a 100mi ride over "dead-mans-pass" and some newbie collapses on the side of the road, is the organizer liable?
-A major issue with my club's web-based rides is that the as the only way to communicate is via email to the organizer but this is clumsy. You want to be able to easily communicate info before or during the ride. Eg you signed up but are running late and want to inform the group or the lunch spot is closed so you're changing venue. This should push to your bike's device (eg Garmin/Wahoo) so you see it while riding.
-a criticism of the cyclique app is you need to sign up just to take a peek. Due to the above posts, I wanted to check this out but don't want to sign-up just to kick the tires.