That chess.com member invited the most new members onto this site, that's what it is.
What's a refferal?

A yellow apple and a red apple, I answered it first and you're incorrect. The individual who accepts has to be registered as a new member and have been bought onto the site by the member who sent the invite.

I have two questions:
1. Can already registered user to add smb as refferal?
2. What does the refferal points gives?

You don't just have to e-mail to get a referral. You can also use a link anywhere (such as your website if you have one). http://www.chess.com/?ref_id=#### ; where the #### is replaced by your member #.Or you can even use a widget on your site (which includes the referral link). You can find your ID on the widget page.
You can also get a referral if someone signs up and puts in your name in the field on the sign-up up page for "referred by" (don't recall if that is what it is actually called).
If someone uses your link to get to the site and signs up while on the session, you will get a successful referral. If a member you referred buys a premium membership, you will get a 1 month free membership (gold I believe).
I just wanted to know, that's all. I saw some players with trophies about it but didn't know what it was.