Here's a quick code snippet you can use to programmatically create new users in Drupal 6.x:

<?php
$newUser = array(
  'name' => 'username',
  'pass' => 'password', // note: do not md5 the password
  'mail' => 'email address',
  'status' => 1,
  'init' => 'email address'
);           
user_save(null, $newUser);
?>

And, here's how you can update an existing user:

<?php
// load user object
$existingUser = user_load('USERID');

// update some user property
$existingUser->some_property = 'blah';

// save existing user
user_save((object) array('uid' => $existingUser->uid), (array) $existingUser);
?>