Saving custom fields for nodes and users
I haven't done this with user_save() before but if it follows convention from the rest of the system (which is likely) it'll be like this:

$values = array(
  'name' =&gt; $name, 
  'pass' =&gt; $password,  
  'mail' =&gt; $email, 
  'status'=&gt;1,
  'field_foobar' =&gt; array(
    LANGUAGE_NONE =&gt; array(
      0 =&gt; array(
        'value' =&gt; $field_value
      )
    )
  )
);

$newUser = user_save('', $values);


In fact there are a few examples further down the page in the user_save() docs confirming this method is the right one.