Extend router network using Raspberry Pi 3 as access point

Question

I currently am using this guide to create an access point using the RPI3 to create a new network, but what I would like is to extend the network to which the RPI3 is connected via Ethernet so that it acts as the same network, i.e. devices on the networks should see each other. Specifically, I would like my Chromecast to be connected to the RPI3 network and have it be controllable on the original network from the main router. Thank you!