Good question. And one that we wrestle with often. The Periodic Table of Office 365 is a representation of the applications that make up much of the Office 365 ecosystem.
Regardless of how users encounter Office 365 Groups (the underlying technology behind using Outlook Groups, Yammer Communities, and Teams Teams), "Groups" isn't an app; it's a membership service.
For a long time, we've left Office 365 Groups off of the Periodic Table. Version 2 still does not include them.
However, we have decided to add Groups to the new Version 3 Beta. It took some finagling to make it work, but we think we've reached a point that's worthy of publishing.
At the same time, we have a great resource on using Office 365 Groups already. Check out https://jum.to/Groups for an everyday infographic on Office 365 Groups.