BuddyPress.org

Network activate it. That’s all you do. Then it runs on the MAIN site.

If you want the Buddypress home base to be on a different site, you need to add a line in the wp-confgi.php file to tell it to do so.

define ( ‘BP_ROOT_BLOG’, $blog_id );

And replace $blog_id with the actual IDd number of the site you want it to run on.