As far as I know, if you try to insert new users and pass them an ID, wordpress tries to update the user with the given ID.
See the Example in the reference: http://codex.wordpress.org/Function_Reference/wp_insert_user#Examples or see the source of the function: http://core.trac.wordpress.org/browser/tags/3.3.2/wp-includes/user.php#L1260 on line 1260, where it checks for the user-ID.
They use wp_insert_user with a given ID for updating ... so the wording is a bit confusing.
In the code from EL45 at pastebin the ID ist set. Remove the ID from the code.
Right now I added lots of Users in WordPress 3.1.1 with the following code:
$user_data = array(
'ID' => '',
'user_pass' => wp_generate_password(),
'user_login' => $loginName,
'display_name' => $loginName,
'first_name' => $firstName,
'last_name' => $lastName,
'role' => get_option('default_role') // Use default role or another role, e.g. 'editor'
$user_id = wp_insert_user( $user_data );
and I set up a default password to the user.
There is no need to set up an email address.
Required are at least user_pass and user_login but I didn't tried this out.