If you use GUI based FTP client software, usually you can specify a default folder when configuring a new FTP site (or connection).
If you use a command-line FTP client, then it usually defaults to the root folder in "My Storage". If you have a group account, you can use the Group Options feature to set a default folder after logon. (However, you can only choose a few options.) In your case, you can set it to:
- Group member default to "go to the sub user shared folder".
With this option set, you will have a shared folder name in your account that matches a sub-user name for each sub-user. This way, when a sub-user logs on, it will default to the shared folder: