An offer to bring up to three companions along at a discount sounds like it must involve registration, group tickets, or some step taken in the app beforehand. Families and anyone hosting visitors hit this most often, usually while working out whether everybody needs a subscription of their own.
It's built into most NS Flex subscriptions, including Dal Voordeel, and it's simpler than it sounds: up to three people traveling with you during off-peak hours get the same 40% discount on their own fares, automatically, with no separate registration or app step needed beforehand. The requirement is that you actually travel together — check in around the same time and place as your companions, ride the same train, and the discount applies to each of their taps just like it applies to yours. It only works during daluren, the same off-peak windows your subscription discount uses, so a group trip during weekday rush hour won't trigger it. Handy for family visits or bringing friends along on a day trip, since none of them need their own subscription to get the reduced fare — just their own OV-chipkaart or OVpay-linked card.