Alert admins about External Sharing limitation in SharePoint Admin Centres
The only way to get external sharing to work on a modern sharepoint team site collection is via the PowerShell command "Set-SPOSite -SharingDomainRestrictionMode AllowList -SharingAllowedDomainList "<list of domain names>" -Identity $site.Url"
(this command includes whitelisted domains for extra security)
Microsoft need to put up a NOTICE in the admin centre, and in the new Preview admin centre that the UI's don't not work for external sharing, and that PowerShell is needed. I have just had this limitation confirmed by a Microsoft Engineer.
Tony Cosentino commented
In addition, it needs to be made clear that external sharing is controlled via the SharePoint Members group. In other words, external/guest users are added to the SharePoint Members group once they join the site.