It actually isn't an app, it's a web page which is only available through the ship's wifi. The link for it is given to you either with check-in paperwork or in the paperwork in your cabin (can't remember exactly). You don't have to use it for dining, but it does help and is easy to use and actually very useful. As for sharing taxis, I would think that using a taxi in Norway is very expensive even if sharing. Sharing taxis is more common in the Mediterranean and Caribbeann. You don't have to take the official tours on any cruise but I would recommend some of them in Norway.