BuddyPress.org

@imjscn — You can put BP on a secondary blog by adding define ( 'BP_ROOT_BLOG', $blog_id );, or in all blogs by adding define ( 'BP_ENABLE_MULTIBLOG', true );, into wp-config. However, by activating the BP at all blogs will make the blog posts on each blog will not appear in the activity.

For more can be found at https://codex.buddypress.org/buddypress-site-administration/changing-internal-configuration-settings