The multi-user (MU) feature has been rolled into core as Multisite ever since version WordPress 3.0.
It is ideal for a central system administrator to set up a large network of blogs with one installation. The sites in this network have different URLs.
Individual, separate site administrators are able to securely manage their templates and settings without affecting the other users.
add one line to wp-config.php
define( 'WP_ALLOW_MULTISITE', true );
login again – follow instructions
Note: Install WordPress in the root of your webpath if you want to use a sub-domain install. A sub-directory install creates a path-based network, even though it does not use file system directories.
add six more lines to wp-config.php
create the hidden .htaccess file containing the rewrite code