In Pokémon Go, you can have other trainers as friends, to keep track of when they are online and playing other games from Niantic. You can add friends via your trainer code, QR code, or you can add friends via Facebook.
Here is how many friends you can add in Niantic's Pokémon GO
How Many Friends Can You Have in Pokémon GO?
Before this past week, you were only able to have up to 200. Now, according to Niantic Support, you can have up to 400 friends in Pokémon Go. Additionally, if you add a friend in Pokémon GO, you'll be adding them as a friend in all Niantic games you and your friend both play.
With friends, you can play together and catch Pokémon in Pokémon GO and play other games together. If you invite friends to Pokémon Go and they join, you can also earn rewards as they play and reach milestones, including 100 free Pokéballs.