Beste antwoord door PPP
I'll try to make a step-by-step howto here. If there's a demand for it, I could make a more detailed one with pictures later. For now, I combine the graphics on the website Tochjo linked in his response with my own experience.
As Tochjo explained, you will need an OV-Chipcard for each of your fellow traveler (naturally, they can use their own if they have one, but disposable chipcards cannot be used for this).
Approach any NS-machine (tickets or "OV-Chipcard holders", doesn't matter). If the machine blabbers a welcome screen in Dutch, tap the union flag at the left bottom corner to switch it to English.
Hold the OV-Chipcard of the person traveling along with you in front of the OV-Chipcard reader on the right side of the machine, below the payment terminal (indicated with OV-Chipcard logo). The machine will read the card and will soon pop up some options.
Choose 'load other products', and from the sub menu choose 'Joint-journey discount (Samenreiskorting)'. In some occasions, also class needs to be specified (1st or 2nd class).
Repeat this step for EVERY time your friend travels along with you. So for your trip back, even on the same day, you will need to repeat these steps again.
Also, make sure you have enough credit on your OV-Chipcard before traveling. For traveling with discount (this includes travel-along discount) you must have at least €10 balance on your card.