B2B and SPO

Hi Jakob

According to this doc this is by design https://docs.microsoft.com/en-us/azure/active-directory/active-directory-b2b-faq. See the question about SPO people picker:

Are B2B collaboration guest users visible in SharePoint Online/OneDrive people picker?

Yes! However, the ability to search for existing guest users in the SharePoint Online people picker is OFF by default to match legacy behavior. You can enable this using the setting 'ShowPeoplePickerSuggestionsForGuestUsers' at the tenant and site collection level. This can be set using the Set-SPOTenant and Set-SPOSite cmdlets, which allow members to search all existing guest users in the directory. Changes in the tenant scope do not affect already provisioned SharePoint Online sites.

So all you need to do it to run the above mentioned cmdlets to see the added users in the picker.