Using get_userdata() in plugin referenced by AJAX.

I’m writing a shoutbox plugin that uses AJAX to make calls to my shoutbox.php and retrieve content.

I’m able to use get_userdata() in functions that echo text into the content, but when AJAX calls other functions from the same file that call get_userdata(), I get
Fatal error: Call to undefined function get_userdata()

I’ve place the following lines at the top of my plugin

if (!function_exists('add_action')) {
    require_once(get_option('siteurl')."/wp-load.php");
}

but I still can’t seem to make it work. Any ideas or workarounds?