I don't know about the app, but on the web you go to the section "book using a discount pass", then select your pass & the number of passengers. Once done, search for your tickets, and once you have selected flights and you need to enter the personal details, you will be prompted for the number of the discount pass for your fellow passengers.
As the app is often way behind the website in terms of functionalities, it's possible that you can only book tickets for yourself on it (until not so long ago, you could not even book a discount pass ticket at all in the app). Maybe someone knows better on that one!