There are two self-capacitance buttons available on the ATtiny817 Xplained Pro board, which can be used as input buttons for an application. These QTouch buttons are intended to be driven by the built-in Peripheral Touch Controller (PTC) of the device.
To get started using the QTouch buttons and the PTC, open the QTouch Tiny817 Xplained Pro Selfcap Example in Atmel Start (http://start.atmel.com/#examples).