CBSubs does not provide any sort of referral system. I suppose you could generate a unique code and add it to a field using CBSubs SQL Actions. Then have a field at registration which users input a referral code. You then check that field with CBSubs SQL Actions and add a respect tick to another field for the referring user. It's very possible, bit complex and some trial and error but certainly possible. Example queries as below.
[cb:if referredby!=""]UPDATE `#__comprofiler` SET `cb_referrals` = ( `cb_referrals` + 1 ) WHERE `cb_referralcode` = '[referredby]'[/cb:if];
UPDATE `#__comprofiler` SET `cb_referralcode` = CONCAT( [user_id], SUBSTR( MD5( RAND() ), 1, 12 ) ) WHERE `id` = [user_id];
The above assumes you have 3 fields: cb_referrals, cb_referralcode, and cb_referredby. Please note the above is UNTESTED, it may require adjustments. Please test thoroughly. You would use the above in the activation field of your plan of CBSubs SQL Actions integration.
Easier approach is to just use PostAffiliatePro or IDevAffiliate which we provide integrations for and are full featured affiliation software.