Strava already has the ability to plan group rides/runs, associated with a club. You have to be one of the admins though.

Club -> Add Club Event

Specify time, meetup location, route, repeat interval, etc.

No - it has to be more casual.

I don’t want to force people to join a/my/some club. I want to let my followers and friends see I plan to do a 80km ride Saturday morning and let them simply tag along. It could easily be integrated into the feed or as a separate section.

I’d argue social activity is one of the biggest opportunities for Strava to lock themselves in as the hub of all movement activity - but it requires supporting how people like to naturally organize in a local community.

If it's that casual, you could post a message on your feed and tag people you want to be notified.

While true I think a more structured way of planning group rides seems very useful and like low hanging fruit for Strava.