Maybe the simplest way is to make a quick plugin that has your own form for inserting users.
wp_insert_user()
doesn’t require an email address or password, and doesn’t generate an email notification.
function wpa70409_add_user(){
$userdata = array(
'user_login' => 'testuser',
'display_name' => 'Test User'
);
wp_insert_user( $userdata );
}