Reference user on new register

Hello guys,

I'm trying to do a user referral on the Register page. For example:


That 'Referral' new field, should be the user of the friend or contact, that was invited or presented the website for the new user. After the Register this new user must be the new friend or contact of the Referral name. Suggestion?

PS.: Ive tried to do this with Profile_Manager without success, 'cause it just save the word and don't add the friend on the contact list.