BuddyPress.org

Your second WP install in subdomain should be installed in the same database (with different prefix) with your first WP install. Btw, backup your database before trying this. So in wp-config.php of the second WP install, just before “That’s it, you can stop editing” you should also add:
define(‘CUSTOM_USER_TABLE’, ‘wp_users’);
define(‘CUSTOM_USER_META_TABLE’, ‘wp_usermeta’);

Having said that WP 3.0 is around the corner, when you upgrade you can make your install multisite and define BuddyPress in subsite in subdomain or subdirectory structure, and you have one log-in for members to access both areas.