Battle parties allow Pokémon GO players to choose up to six Pokémon to quickly select for Gym and Raid Battles. You might have a favorite set of Pokémon that you always bring to battle with you, or maybe you have a few Pokémon that deal significant damage against Raid Boss Pokémon. Battle parties make selecting your preferred Pokémon fast and easy.
To create a Battle party:
- In Map View, touch the Main Menu button.
- Touch the Pokémon button.
- At the top of the screen, touch the Party tab.
- Touch the Create Party button.
Once you have selected your Battle parties, join a Gym or Raid Battle to use them. Choose your party by swiping to the left over the recommended Pokémon list.
Note: This feature is currently in development. Some functionality may change. While in beta, Battle parties preferences will reset if you switch devices or reinstall the app.
Source: Official Pokémon GO support page