Guild chat is the place where you can discuss tactics with your guild mates or simply ask them for help.
Tap on the little arrow on the right side of your dungeon screen to enter guild chat.

If you have selected an age of 1-16 on the start screen, social features such as Facebook and Guild Chat will not be available. Unfortunately, we cannot turn them on, but you can reinstall the game, create a new account and choose a different age.