How to Add a Guest User to a Group

Any group member can nominate an Office 365 group external user for guest access, but only the group owner can grant guest access. The process of adding a guest user to a group proceeds as follows:
The group owner or a group member uses the Groups > Add Members command to nominate the external user for membership by entering the user’s email address.
The group owner reviews the access permissions the guest would receive by joining and approves the nomination.
The guest receives a welcome email and can begin participating in group activities.

Guest members of a Microsoft 365 group:
Don’t have direct access to any of the group’s sites, such as a team site in SharePoint
Can participate in group activities through conversations and group calendar invitations sent to their email inbox
Can access shared files included in email messages, such as attachments or links, provided the administrator has enabled the requisite file-sharing permissions

The external sharing capabilities of SharePoint Online can be managed at two levels:
Across the entire Microsoft 365 tenant, through either the SharePoint Admin Center, the Microsoft 365 admin center or Azure AD
At the site level

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