WP functions in "side" plugin files

Flyer, the only way to do that is include wp-load.php… however, I think it’s best to make your new file and include it in the main, that way you can use WordPress’ functions without loading resources twice.

Example:

<?php
     /* FILE: mainPluginFile.php */
     include('userFile.php');

     /* The rest of your code here */
?>

<?php
    /* FILE: userFile.php */
    function userData(){
         /* Do your code here */
    }

    add_action('init', 'userData');
?>

You might want to also look into the get_currentuserinfo function which you can wrap inside the is_user_logged_in method.