Is it possible an application with app-only permissions to change the availability (Presence's state) of group users?
Reading the known issues for "Group conversations, events" delegate permissions are needed. So it seems not possible.
Any other way?
(BTW UCWA is not a way as it needs also user's credentials AFAIK.)
At the moment there is no support for Skype or Skype for Business within Microsoft Graph. I recommend visiting the UserVoice and adding this suggestion.
You can find the current set of Skype API's at the Skype Developer Platform site. I think you may be looking for the Trusted Application API (Public Preview) which brings a lot of the UCMA functionality to Skype for Business Online.
