To earn 5€ from the promo, you will need to invite your friends from the "Bring friends" section in the payzy app. With every successful registration, you will earn 5€ and your friends will earn another 5€! All you need to do is tap here https://link.payzy.gr/baf_land on your mobile phone and start inviting friends!
The main requirement for you to earn the 5€, is for your friend to register via the link you forwarded to them and within the duration of the promo (see here).