You can now assign different internal and external access levels for leads. Use external org-wide defaults to set a more restrictive access level for external users without changing the access that internal users have.
Where: This change applies to Lightning Experience and Salesforce Classic in Professional, Enterprise, Performance, Unlimited, and Developer editions.
How: To set the external access level, from Setup, enter Sharing Settings in the Quick Find box, then select Sharing Settings. Under Organization-Wide Defaults, edit the default external access for Lead.