Assuming you can achieve this easily installing a Chromecast behind the TV is simply not enough. The existing Wi-Fi network must be adapted to create a safe connection between the Chromecast and the phone or tablet. We want to prevent that your guest accidentally streams its content to the TV of the neighbours. Imagine that!
Sure the Google Chromecast has a guest mode that according to the manual would be deployable in a small hotel. Sadly, the practice teaches us that this does not work out great to put it lightly. For example, Netflix (one of the most wanted functions) would not work being in breach of their license terms.