Filemaker files are opened with Account names and passwords, not with user names. But Filemaker automatically enters the current user name into the account name box in the password log in dialog. This can create a lot of confusion as to whether you are working with a user name or an account name.
Account names are created in Manage | Security and each needs to have a matching password (which can be blank).
User names, as you described, are created in preferences.
When you log in, even though FileMaker enters the current user name for you, you can still clear this name and enter a different account name.
Our systems here enter a user name for the most frequently used limited access account (we don't manage separate accounts for each user), but supervisors can clear the default account name entered from the local user account and can instead enter their higher access level account name and password.