how to use the FTP with clients?

Hello experienced users!

I am having trouble finding answers to the ftp services on WordPress.

Here is the project:

I am trying to build something that will allow our company to upload files/photos up onto our WordPress site so that our client can view them instead of sending them through emails. And each client will have their own secure log-in and they don’t have the access to other clients’ files.

Anybody know how? Thanks in advance!!!