Iveta, it's hard to say, but worst that can happen probably is you don't get paid $15. However, this part violates Upwork ToS:
**Since we need a large number of users, we are opening the door for referrals, you can refer users from your friends, colleagues or family.
** They don't need to be Upwork users, payment will be done to you.
**Divide the 15$ between you and the referral as you please.
Because they found you through Upwork, they have to pay you through Upwork.
"No good deed goes unpunished." -- Clare Boothe Luce