Playing with a Buddy in Pokémon GO is a necessary step to completing all the tasks assigned players in the currently ongoing Johto Throwback event, in which Pokémon from that region are highlighted. But it's not immediately apparent how to play with friends in Pokémon GO. Here's how to do it.
Play with Buddy Pokémon GO: How to Play with a Buddy to Complete the Task
To play with a Buddy in Pokémon GO, you'll first have to select a Buddy Pokémon. Tap on the Trainer portrait at the bottom left corner of the screen and then select "Choose Buddy." From that screen, select the Pokémon you want as your Buddy.
Choosing a Buddy will allow you to access its Buddy Profile by tapping on its portrait. In the Buddy Profile, selecting the "Play!" button below the Pokémon will activate your phone's camera, placing your Buddy Pokémon in your environment.
Once you've got your Buddy Pokémon in your environment, you can play with it by rubbing it back and forth, causing sparkles to appear on the Pokémon's body, followed by a heart over its head. As soon as you see the heart, you'll have completed the task.