Iis FTP strategy. One or multiple FTP site?

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FTP

IIS7.5

This is the first time I try to set up FTP folder with IIS. I try to understand the concept and the correct strategy to apply.

First, I need to create several FTP access for different users. These users are complete different companies. I also need to give a read only access to all these FTP folders to one process.

My question is: should I create one FTPsite with several subfolders, one subfolder for each company or several FTPsite for each company?

What I have tried:

At this moment in our company the structure is one FTP site with several subfolders. I don't like it because

1. I need to create a user in our domain to give access to the subfolder.
2. It seems the subfolder must have the same name than the user. (EDIT I confirm this user isolation level is set to User name directory)

Point 2 is blocking for me because I want to create multiple user with same root FTP folder (EDIT perhaps I can just change isolation level).

I have the feeling that this is wrong. i cannot let different companies connect to the same FTP, with the same root security level, with the same SSL probably (at this moment we dont use SSL), I don't know. I'm not a server admin.