To play co-op mode in Genshin Impact, a player must first reach Adventure Rank 16. This can be achieved by completing quests, opening treasure chests, finishing entries in the Adventurers Journal, and exploring the world. Once a player reaches Adventure Rank 16, co-op mode will automatically unlock and can be accessed through the main menu. However, players can only play on the lowest World Rank they have in common, and each player must have at least Adventure Rank 16 to unlock the co-op feature.