You receive Atlas Credit when someone creates an account using your referral code. This means that if a Client of yours does not have an Atlas account, and he/she creates one, then you will get Atlas credit. You can also share your referral code with people that want to become a Specialist and create an account on Atlas.
Your Clients or acquaintances/friends that already have an Atlas account can’t add your referral code to their account. You can only add the referral code upon creating an account.