What do you mean by adding another traveller?

If you made a travel plan/itinerary in the tab "My Trip", you can share it with someone. Press the three dots on the top right corn, and select "share as link". Share and you both have the same itinerary.

Note, it doesn't update on the other device if someone adds changes. 

If you want to add another pass, then go to My Pass and tap on the big + at the bottom right.

If you want to book reservations for multiple people, that's independent of the app.