With Portal it's possible to allow automatic registration of enterprise accounts (Automatic registration of enterprise accounts—Portal for ArcGIS (10.6) | ArcGIS Enterprise ), but I can't find if it's possibile to define a default group where these users can be assigned. This can be useful when you want to share some specific content to several users, and do not want to bother with registration or have users login to the Portal to join the group.
This would definitely be useful functionality. In our case we would want automatically registered users to be given membership of a number of different groups that contain various data layers. The groups would then be available to them on the Add Data button within the web map viewer to help make browsing of data easier.
I guess it's something that might be possible with the new webhooks functionality that is available in 10.7?