External users can't find users in the people picker in SharePoint Online
An external user tries to share a Microsoft SharePoint Online resource with a Microsoft Office 365 user. But when the external user types either the other user's name or a partial email address, the people picker doesn't return results.
To work around this issue, external users must type the complete email address of the internal users to share anything within SharePoint Online with those users.
This issue occurs because external users can't run Active Directory queries in the people picker in SharePoint Online.
Still need help? Go to Microsoft Community.