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.

@seanbaugh What I posted above was for two single WP installs. I have to add that if you’re going to do the above, make sure that the 1st WP install and the 2nd WP install have the same versions. So if existing install is WP 2.9.2, then your second install must also be WP 2.9.2 – can’t guarantee it will work if you use different versions. Just backup your 1st install’s database first if you want to experiment with different versions.

@chucksweatt