Max Raid Battles in Pokémon Sword and Shield allow you to connect with friends to battle together via Nintendo Switch Online. If three other Trainers aren’t available to participate in Max Raid Battles, support Trainers will automatically be added to the player’s team to help battle the wild Dynamax Pokémon. However, a Nintendo Switch Online membership is not required for an in-game purchase option:
Nintendo Switch Online membership (sold separately) and Nintendo Account required for some online features. This game includes an in-game purchase option for users who do not have a membership. Not available in all countries. Internet access required for online features. Terms apply.nintendo.com/switch-online.
Source: swordshield.pokemon.com
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