BuddyPress.org

Should have tested my function first as it didn’t work. This one i’ve tested and it does work:

function custom_menu_item ( $items ) {

   if ( !is_user_logged_in() ) {
      $items .= '<li><a href="/register" title="Register">Register</a></li>';
   } 
   
   return $items;
}
add_filter( 'wp_nav_menu_items', 'custom_menu_item' );