To start, take a look at the "account" script steps listed in the script editor. They can be used to make matching changes to accounts and passwords for multiple files so that each one gets the same set of accounts and with the same associated passwords. You can set these scripts to "run with full access permissions" so that users who do not have a full access password can run these scripts to manage accounts.
You could set up a single table, either in an existing table or as it's own file to use with these scripts to manage account names, privileges and passwords, though for security purposes, you might choose not to store actual passwords in the table--either encrypting them or not storing them at all. (Users who forget their password can have their password "reset" with an option to enter a new password to avoid having to store actual passwords.
You also should read up on "server side authentication" as an option for using the Server's OS to manage access to the files instead of FileMaker.
Thankyou very much for your answer and the info attached. We didn't realize such topic was already developped.in the instructions. We will be back to you for more specifc questions.