This is a small code I wrote to batch create generic users

<?php $lock = true; //true = disabled (locked)

if(!$lock) { // if not locked
    require_once( $_SERVER['DOCUMENT_ROOT'] . '/wp-load.php' );

    $last_registered_user = $wpdb->get_row("SELECT ID FROM $wpdb->users ORDER BY ID DESC LIMIT 1");
    $new_id = $last_registered_user->ID + 1;
    $limit = false; // extra lock

    while($new_id <= $limit) {
        wp_create_user( 'user_'.$new_id, 'changeme', 'user'.$new_id.'@example.com' );
        $new_id++;
    }
} else { // if locked ?>
    <h5>This script is locked to prevent accidental use.</h5>
<?php
}
?>

Setup

A couple things to do:

1. Change unlock the script by setting $lock to false.
2. Change the path to wp-load.php.
3. Change $limit to the number of users you want to create.

Use

This script will create $limit number of users with the username user_andTheirID and their e-mail address [email protected]. To edit the username, once the users have been created, you will have to edit the database itself.

Extra

If you want users to change their passwords from the default, changeme, try the Force Password Change plugin by Simon Blackbourn. This will require users to change their password on their first log in. Make sure you install this plugin BEFORE you run the batch create, otherwise the right values will not be created for the existing users.