$user_id in plugins

i’m trying to access the current user’s id in a plugin. the plugin is called here:

add_action('admin_menu', 'add_guest_pages');

i’ve tried get_currentuserinfo() but i get a “function undefined” error. i’m assuming tha’ts because pluggable.php hasn’t been called yet.. but i still need to get my user’s id..

how can i get my user’s id from my plugin?

help is much appreciated. thanks.