Sending (e-mail) updates to App users

I recently moved from distrubuting dashboard/reports through workspaces to Apps. 

Access to these apps is sorted by AAD security groups (which are automatically filled). 

All this makes my life a lot easier, which I appreciate.

Now I want to send all these App users some info (about the use of the bookmarks in Power BI).

In the past I would do this by sending a mail to the members of the Workspace, but since the App users aren't in a workspace anymore, but only in the AAD security group, I cant use this route anymore.

Is there a way to send an update to all users, either from within the app, or maybe create a pop-up window within the App that shows up automatically when people open the App?