The quick way for users to understand O365: The Periodic Table of Office 365

Office 365 Groups are a crucial component of Office 365 underpinning many services as well as a function in their own right and arguably the successor to Exchange Distribution Lists:

You can see how https://docs.microsoft.com/en-us/MicrosoftTeams/office-365-groups for example and more in general https://support.office.com/en-us/article/Learn-about-Office-365-groups-b565caa1-5c40-40ef-9915-60fdb2d97fa2.  Definitely well worth covering groups in your governance plan, for example, looking at https://support.office.com/en-us/article/manage-who-can-create-office-365-groups-4c46c8cb-17d0-44b5-9776-005fced8e618, how they are managed and whether the capabilities of https://www.petri.com/aad-licensing-groups-teams could be beneficial, as that provides a lot of extra features for groups management.

These presentations may be worth checking out as well:

Implement best practices with Office 365 Groups

Governance and compliance with Office 365 Groups