FTP Site Security

I’m new to FTP and could use some help.

I just “inherited” an FTP site (running on IIS)that management would like to secure. The site is used to transfer files between us and clients. The site has a username and password for access but once you are in, all folders on the site are accessible to everyone. Is there anyway to put security on each folder that is on the site? For example if I have 20 folders on the site can I secure each folder in a way to allow “client A” access to one folder but not the other?