A Microsoft file hosting and synchronization service.
Hi The Razor,
We cannot add external users to Office 365 Groups. But we can add the external user accounts to a SharePoint group to grant access permission: in site settings > site permissions > Create Group in Permission tab > click Grant Permissions > type in the external user account > Share > by default, this invites the users who you add to join the SharePoint Members group. To choose a different group and permission level, click Show options and then choose a different SharePoint group or permission level under Select a group or permission level.
Thanks,
Linda