In WPMU, there was an option that would let you add Buddypress function on allmulti-blogs (network blogs) installed under the main installation. Due to the big merge in WordPress 3.0, now you can enable BuddyPress in WordPress using the wp-config.php file as well.
Simply open your wp-config.php file and add the following line.
define ( 'BP_ENABLE_MULTIBLOG', true );
Note: Do this only if you want Buddypress to show up across all blogs in the system. It’s not required.