If you have this feature on your hosting package you can manage your different FTP accounts from your control panel.

You may add FTP accounts allowing full access to your directories or restrict access to a certain subdomain (also setup in the cPanel for the account).

Please remember that any additional FTP accounts setup for subdomain access will need to login with a slightly different username format. The domain must also be used with these FTP logins using the following format:

username@domain.com

username = username setup in cPanel when adding the FTP
account domain.com = your actual domain name