add new user to access sub folder of existing domain

I currently own a web site and recently added two more accounts.

I would like user 2 to have have admin privileges to public_html/subdir1.

My account has a star on the people icon and the others do not.

I am assuming that this is a privileged account with full access.

I want to set up a FTP account in Filezilla for the two new accounts to access.

How do I go about doing this?