Office 365 Groups is a service that works with the Office 365 tools you use already so you can collaborate with your teammates when writing documents, creating spreadsheets, working on project plans, scheduling meetings, or sending email.
Shared Inbox– For email conversations between your members. This inbox has an email address and can be set to accept messages from people outside the group and even outside your organization, much like a traditional distribution list.
Shared Calendar – For scheduling events related to the group.
SharePoint Document Library– A central place for the group to store and share files.
Shared OneNote Notebook – For gathering ideas, research, and information.
SharePoint Team Site– A central repository for information, links and content relating to your group.
Planner– For assigning and managing project tasks among your group members.
Reference: Learn about Office 365 Groups