To share user data across multiple installtions of WordPress you need to use the same database as affix different table prefizes in wp-config.php for each blog or install.
Then you need to add these lines of code to the wordpress installs that are NOT your main install (i.e. on the ones that are the addons if you like and where people WON'T register).
Change the words my_users and my_usermeta to whatever the table prefixes are in the main install (i.e. the one were everybody registers).
If you want to make sure that users are kept logged in to all installs when they have logged into the main site then add this code underneath the above code:
$public_cookiehash = md5('http://www.ralphmoorehawaii.com/');
define('USER_COOKIE', 'wordpressuser_'. $public_cookiehash);
define('PASS_COOKIE', 'wordpresspass_'. $public_cookiehash);
Replace the URL with the url of your main install. Hope that helps.