How can I import users into WordPress?

A new file, import.php:

<?php

define('WP_INSTALLING', true); // this is required. i forget why!

include '/path/to/wordpress/wp-load.php';
include ABSPATH . WPINC . '/registration.php'; // wp_insert_user()

$userdata = array('user_login' => 'jdoe', 'user_pass' => 'foobar');
$user_id = wp_insert_user($userdata);

Check wp_insert_user() for other possible fields. Run update_usermeta() for any additional needed meta fields. (Including user level, though there may be convenience functions.)

Note that here at work we redefine the wp_authenticate() function (it’s in pluggable.php, so it can be replaced by defining it in your own plugin) and creating user accounts on-demand if they don’t exist at login time.