Microsoft provides good starting guides at the following sites:

Get Started with Office 365 Groups: https://support.office.com/en-us/article/Get-started-with-Office-365-Groups-in-Outlook-B86C141B-39CF-49D9-A4DB-124C3D786204

Learn about Office 365 Groups:  https://support.office.com/en-us/article/Learn-about-Office-365-groups-b565caa1-5c40-40ef-9915-60fdb2d97fa2 Office 365

Use Groups in the Outlook mobile app: https://support.office.com/en-us/article/Use-Groups-in-the-Outlook-mobile-app-7804076e-0acc-48f6-87a1-083b98a5ffa2