The introduction of Office 365 groups gives the rich collaboration within a team inside an organization. There will be a situation you need to give access to outside of organisation when you work closely with customers, partners, and others outside organizations. The importance of collaboration with external users is inevitable in various business needs, to resolve this, Microsoft finally announced the external user (Guest) access feature: Introducing guest access for Office 365 Groups
Note: Microsoft announced the guest access feature works only for
email accounts including corporate and consumer domains (such as Outlook.com or Gmail.com). If the guest email identity is associated with a Microsoft account (such as Office 365 or Outlook.com accounts, for instance), the user is directed to a sign-in page to identify themselves. If the guest doesn’t have a Microsoft account, they will be directed to a sign-up page to create an account.
You can also read this great article https://www.petri.com/external-access-office-365-groups (by Tony Redmond) to know more about how to add guest users for Office 365 Groups and how an external user invitation works for guest users.