Filetable: directory permissions

With a FileTable, SQL Server performs authorization checks based on the user’s Windows account. The Windows account must have permissions to connect to the SQL instance and database along with permission on the FileTable, just like when the table is used directly in T-SQL. These permissions can be assigned directly or via Windows group membership

CREATE LOGIN [YourDomain\WindowsUserOrGroup] FROM WINDOWS;
CREATE USER [YourDomain\WindowsUserOrGroup];
GRANT SELECT ON dbo.YourFileTable TO [YourDomain\WindowsUserOrGroup];