Pokemon GO has many active players, but there are also plenty of players that haven't picked up the game in a while. In order to promote inviting back older players and bringing in new players, those who refer these players into the game can earn a variety of valuable rewards.
Every current Pokemon GO player has the ability to give out a referral code that can be used by those joining the game or players that have been inactive for at least 90 days. Players that enter a referral code will get 100 Pokeballs immediately as a reward with both players also able to get even more prizes as the new or returning player completes certain tasks.
Prizes from the referral code can be accessed through players' friend lists. Only progress made by the new or returning player will affect when rewards are to be earned by both the new or returning player or the code giver.
At this time there are fifteen different tasks that new or returning players can complete to earn prizes for both themselves and the code-giving player. While some of the rewards are the same for both players, there are many prizes that are different depending on who is the code giver and who is completing tasks.
The tasks that new or returning players can complete to earn rewards for themselves and the code giving player are:
Defeat three Team GO Rocket Grunts - Charizard encounter
Catch 1,000 Pokemon - Three Incenses (Referrer)/Three Premium Battle Passes (New or Returning Player)
Earn 50 Candies walking with your buddy - 30 Rare Candies (Referrer)/Five Premium Battle Passes (New or Returning Player)
Defeat a Team GO Rocket Leader - 50 Charizard Mega Energy
Give your buddy 50 treats - Gible encounter (Referrer)/Five Rare Candies (New or Returning Player)
Win 20 Raids - Five Super Incubators (Referrer)/Three Super Incubators (New or Returning Player)
Visit Poke Stops 30 days in a row - Unknown Pokemon encounter
Trade Pokemon five times - Ten Pidgey Stickers
Catch 50 Pokemon - Darumaka encounter (Referrer)/Super Incubator (New or Returning Player)
Win three Raids - Premium Battle Pass (Referrer)/Lapras encounter (New or Returning Player)
Make two new friends - 10 Pikachu Pal Stickers
Hatch three Eggs - Farfetch'd Encounter (Referrer)/One Rare Candy (New or Returning Player)
Make 100 Nice Throws - Two Super Incubators (Referrer)/Snorlax encounter (New or Returning Player)
Hatch 20 Eggs - Deino encounter (Referrer)/Chansey encounter (New or Returning Player)
Win a Raid - Bagon encounter (Referrer)/Premium Battle Pass (New or Returning Player)
The majority of these tasks aren't meant to be grinded out within a short period of time, rather gotten naturally as the new or returning player goes through Pokemon GO normally. These rewards will definitely encourage players to get back into the game and enjoy all of the newer content that was added in their absence.
Pokemon GO is available on Android and iOS.
Source: Serebii