Hi Christian,
Unfortunately, to the best of my knowledge, it is not possible to send messages through the ArcGIS Online platform to other members. There is a notification system (i.e. you're notified when you have been added to a group), but not a messaging system.
You can use the ArcGIS API for Python to member information for people in your organization (see this blog post by Egge-Jan Pollé). However, I don't think this will work for people outside of your organization.
There was a similar question asked by Peter Timmers which looks like it never got answered
I found this idea about adding a messaging capability to ArcGIS Online. It's a pretty old post, but maybe it can be revitalized with some comments and up-votes. Or if this doesn't cover exactly what you want, I recommend making a new one! If you do, share it here so we can spread the word
Cheers,
Katherine
Best,
Katie
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