Assign training based on 'WHO' the manager is

I have a group that wants to assign training based on ‘Who’ the manager is.

For example all direct employees under Jan Doe would automatically get assigned training.

I’m not seeing in creating groups a way to add field ‘Direct Manager’ 

I also don’t see in custom report that I can filter a report to only display everyone that is under a particular manager or managers.

I am aware of the My Teams and we do utilize this. But curious how anyone handles outside of the direct manager assigning themselves but auto-assigning.

Thanks!