BuddyPress.org

add_action( 'wpmu_activate_user', 'synchro_wp_usermeta', 10, 3);

function synchro_wp_usermeta( $user_id, $password, $meta ) { ... }

You’re also going to need to hook into the ‘wpmu_activate_blog’ action, as activate_user is not called when the user chooses to create a blog on registration.