Configuring WPMU to run BuddyPress on multiple blogs is easier than you might think. This is a tip directly from the BuddyPress guide to changing internal configuration settings:
Allow BuddyPress to function on multiple blogs of a WPMU installation, not just on one root blog:
define ( 'BP_ENABLE_MULTIBLOG', true );
Add that bit to your wp-config.php file and you’re good to go!
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