It is already part of cs-cart. Under addons in the main tab there is the option to start using it or not.
This is the list that I have , but I do not see the refer a friend tab.
Comments and reviews
Customers also bought
News & e-mails
Send the page link to a friend
SEO (requires Apache + mod_rewrite installed)
Make sure the “Send the page link to a friend” is checked.
I just realized that you might be asking about a general referral to the site (e.g., where points/credit are earned) rather than the product referral. CS-Cart does not currently offer a general site referral capability.
This is what I was asking about as well, under “Viral Marketing” thread.
It would be great if you could reward people for referring friends (for example if they signed up to a newsletter or bought a product). Ideally you would be able to allow reward points to accumulate.
It would be even better if you had the option to combine these credits or rewards with the reward points system you already have in the cs-cart. That way it would be possible for your customers to earn free products every once and a while, just from referring their friends.