FTP plugin for client transfers

I want to add a FTP feature to a my clients site so that their initial customers can load files to a customer folder for cost proposals. After the initial customer becomes their client, my client can then assign them a user ID and password for proprietary login and future file transfer, and my client’s administrator can move the initial files.

Is there a plugin available that a potential client can load initial files easily to a predetermined folder without a formal FTP client or administrator rights?