BuddyPress.org

// If user is not logged in and
if (!is_user_logged_in() &&
// The current page is not register or activation
!bp_is_register_page() &&
!bp_is_activation_page() &&
// The current pas is not blog page
!bp_is_blog_page() )
// Redirect to registration page. Change /register to your register page slug
bp_core_redirect( get_option('home') . '/register' );
}

// here, you can add som config paramt to check if private community is enabled
add_action( 'wp', 'restrict_access', 3 );