In case you have any question or problem not discussed and solved here, look for general support or contact me.

Does it support multiple users with multiple accounts
No, in most cases a simple one-account setup is enough. But if you are familiar with the encoded htpasswd you may easily add accounts on your own.

Requirements: Apache, IIS, ...
In order for the password protection to work you must host your website on an Apache Web Server (most popular web server anyway) and it must be configured to support htaccess (which is true in most cases too).

The IIS is not supported by Accessus but in case you have setup this server to support the Apache CRYPT mechanismn it may work too.