I've setup a HTTPS and FTP site in IIS but how can I add users to use the FTP site?
Does they necessarily need an account with password in the computer to be able to use it?
IIS's FTP system uses local or domain users for authentication.
Edited to add: You can also use IIS Manager users in IIS7+ if you don't want to provide 'real' Windows users.
You can set up anonymous users in FTP for IIS. Here is a guide for IIS v6, however there are guides on the internet for all version if you just google.