Blizzard has announced the Recruit a Friend program is going on hiatus June 11, 2019. After that date, you will not be able to send new RAF invitations.

You can choose one of Nine special rewards. These are all account-wide.

Five Recruit A Friend Mount: 

 Cindermane Charger

 Emerald Hippogryph
 Heart of the Nightwing
 X-53 Touring Rocket
 Swift Zhevra

Four Recruit A Friend Pet: 

Golden Pig Coin
Silver Pig Coin
Jade Tiger
Zipao Tiger

Recruit-A-Friend allows you to invite friends and family members to play World of Warcraft. When Recruit successful, you can begin an adventure in Azeroth together with your friends. 
Open up your Friends List and click the "Recruit a Friend" button to send invite email to your friend. Once he accepted the recruit and created a new account, his account will be linked with yours, in which way you can find him in the game easily.
Both the recruiter and the recruit gain benefits for 90 days when they play World of Warcraft together: 
Both the recruit and the recruiter can summon each other while partied. It is more convenient to adventure together.
Both the recruit and the recruiter gain 300% experience points while partied and within 100 yards of each other. To experience the rapid upgrade!
Recruit characters gain one "grantable" level, they may then grant these levels to the recruiter's lower-level characters.
You will receive a free 30 days game credit for your friend who pays for the first month of game time; also you will gain an epic in-game mount or a unique pet, Feats of Strength as a reward.
Tips: You can create a new account to recruit yourself as well, and then you will earn both the recruit rewards and a new character.