Starting March 23, 2018, Microsoft is giving you new ways to govern the access of external users. As part of this update, external users will no longer be able to see content that has been Shared with Everyone, All Authenticated Users, or All Forms Users, even if they have been invited or added to authorized groups.
In SharePoint Online and OneDrive for Business, sharing resources (files, folders, sites, etc.) with external users is accomplished by sending an invitation to the users directly, or adding them to groups and granting appropriate access levels. As a result, an external user would only see the content that has been shared with the user or with groups to which the user belongs.
After March 23, 2018, external users will no longer see content that has been shared with Everyone, All Authenticated Users, or All Forms Users. Only a small number of tenancies use this permissions model to grant access to their specific external invited guests. Content that has been permissioned to these groups will, by default, be visible only to your users within your organization’s tenancy.
If you would like to allow external users the ability to see content that has been shared with Everyone, All Authenticated Users, or All Forms Users within your organization’s tenancy, as administrator, you still have the ability to do so.