Source: http://forum.worldoftanks.com/index.php?/topic/402080-refer-a-friend/

I will just leave this here. And the “secret reward tank” is T95E2 IIRC

You can now invite friends to play World of Tanks with you, using our new Refer-A-Friend Program!

How to Refer A Friend

There are two different types of referrals that you can make:

- A referral for a NEW player (someone who has never played World of Tanks)

- A referral for an INACTIVE player (someone who has not played for at least 60 days)

To invite a player using the Refer a Friend Program, you input either

- Their email address (for NEW players)

- Their username, onto the “Invite Player” section of your Account Management page (for INACTIVE players)

The player that you have invited will need to accept the invitation by following the link that is provided in the email that is sent to them.

The inviting player will be able to see the different invites that they have sent out and whether or not they have been accepted.

You will earn invitations based upon how many games you have played (starting with 100 games required). You will have a maximum of 10 invitations available at any one time. Invites are only regained once a player accepts or declines your invite.

The Refer A Friend Bonus

When a referred player platoons with the player that referred them, both players will receive a boost in experience earned. This amount is dependent on the factors including: when the referral was accepted and how much experience a player has earned on their account so far. You will receive the highest bonus available when platooning with multiple invitees.

Referrals are added to your Friends List. They are easily recognizable by the handshake icon next to their user name in the client.

You can track the amount of experience that your referred friend has earned while platooning with you from your Account Management page.

Rewards for the Recruiter are based both on the amount of experience that the player earns while platooned with you and when playing solo or with other players. You are alsorewarded with the tank when the player reaches and purchases a tier X vehicle (this is for new referrals only.)