Ajax & WordPress Plugin

I am currently writing a roster plugin for rift. I currently have the plugin parsing an xml file storing it in the database and able to pull the data from the database to display via a shortcode.

I am now trying to add a “claim” function for logged in users. I can hack it with just php and mysql but I’d like to use it WordPress style and be able to call WordPress functions – specifically get_currentuserinfo()

I am currently using ajax as the means to doing this. However when I include

<?php
require('/the/path/to/your/wp-blog-header.php');
?>

My ajax displayed modal continues to function. When I remove the include it works fine but gives me an error for the above mentioned function (which I’d expect).

I can use help in one of two ways I guess.
1. Point me in the right direction so that when a user who is logged in clicks the claim button their logged in user id is associated to the roster member they just “claimed”
2. Assist me in figuring out why when I include the file my page stops working.

I know I can write the code to do it php & mysql without WordPress so I don’t need that assistance.

What is the better means of doing this particular function?