Groups in Office 365 let you choose a set of people that you wish to collaborate with and easily set up a collection of resources for those people to share.
Office 365 Groups is a service developed with collaboration in mind.
The workspace comes with a shared inbox, calendar, document library, OneNote notebook, and Planner.
Groups 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. You don't have to worry about manually assigning permissions to all those resources because adding members to the group automatically gives them the permissions they need to the tools your group provides. Additionally, groups are the new and improved experience for what we used to use distribution lists or shared mailboxes to do.