Hi Holger,
You are correct, right now in Dataverse the only way to sync the users/groups would be to hit the sync button in the UI. However we do have an automated sync feature on our road-map for a future release.
In the meantime, what we can do is create a dataflow which performs a sync, this could then be automated using our Scheduler functionality within the product. We can certainly help you out to provide a dataflow which can provide this.
Kind Regards,
Dave