As with all things Microsoft, there are many layers and options. (Feel free to interpret this as you will -- we all do.) For CEI to get the desired functionality from our new sites and not to just turn on every option possible, we chose to create a "Team site" (for each department and subsidiary) AND to connect an Office 365 group to each, for additional functionality. This lets all of us see who is a member of a site, have a group inbox and calendar, etc. It's also for other, less-visible reasons like permissions flexibility on the site, and being able to have "public" (CEI-only) spaces/libraries and locked down libraries, all in one site.
So, the email you received was triggered by you being added to the group that is linked to that site.
This Office 365 group functionality is visible in Outlook, for any group that you're a member of.
The group features include (and have online links to them in your SharePoint site, under the Groups menu header):
- Calendar (can receive/process meeting requests or just be used to show staffing or group events -- scroll down to read more about Calendars vs. Events in SharePoint)
- Inbox (shared, group email or "Conversations")
- OneNote (shared notebook -- handy for keeping track of less-formal info that others in your department may want to see)
Note: If you have been given access to only specific locations or libraries on another group's site, you are not considered to be a member and will not be able to access these features (in that group).