If your friend is going to travel with you together, your friend can go to the ticket machine before check-in. At the ticket machine, your friend can activate the travel together discount. 

The travel together discount is only for when you are travelling with someone who has a subscription by NS. It's not allowed to use the discount both. When you activate the discount for yourself and your friend is doing the same, you have the chance to receive a fine. 

Everything is explained on this page: https://www.ns.nl/en/travel-information/traveling-with-the-ov-chipkaart/combined-travel-discount.html

Welcome to the NS Community @majid13!

What kind of NS Flex subscription do you have? If it’s Flex Basis, you are not eligible to take others with you.

And what @Sander-VH and @mrfreeze already said, your fellow travellers will have to arrange the discount on their own card. You yourself need to do nothing except having a valid check-in with your NS Flex subscription.