Found the answer myself at the bottom of @Eric_Schlange_ZwftIn 's recent excellent ZwiftInsider article about clubs (not allowed to include the link here).
Looks like this is not going to happen anytime soon, even with the clubs expansion 
However note that you can do this, it is just a manual operation:
Join meet-up a few minutes before the start. While sitting there, select a workout from the workout menu. When the meet-up starts, the workout starts automatically. Keep everyone together will work if activated.
People can do separate workouts or just ride freely as they prefer. Most workouts are tied to your FTP estimate so suitable for all levels.
Yes, I already noticed that is an option and will probably try it when everyone has a bit more Zwift experience. But my kids currently have very little experience, so the procedure for setting that up would be too complex.
(Also, based on my experience from last week it appears the āKeep everyone togetherā option works a lot less strict than it used to - it used to be damn near impossible to drop anyone, and it is definitely not anymore. Has anyone tried to ride a meetup with the described āgroup workout hackā recently?)
But in addition, for my āgroup workout meetupsā I would often want to design my own custom workouts to get just the right combination of time and intervals - same as I do outside. Not sure if this is even planned for meetups or clubs, but custom workouts seem to be possible in group workouts in the current version.
We do meetup workouts atleast 4 days a week.