From Bulbapedia, the community-driven Pokémon encyclopedia.
Groups are a feature in Generation IV that allow synchronization of certain events between games.
Players in the same group encounter the same swarming Pokémon, weather conditions, changing Pokémon in the Great Marsh, Feebas locations, Pokémon Lottery Corner lucky number, slot machine odds, and order of Battle Tower Trainers. Group members can compare records on the third floor of Jubilife TV.
Due to Pokémon outbreaks differing between Pokémon Diamond and Pearl and Pokémon Platinum, the Pokémon swarming in Platinum will not be the same as the Pokémon swarming in a Pokémon Diamond or Pearl in the same group. Likewise, Pokémon HeartGold and SoulSilver Mass Outbreaks do not mimic those from Diamond, Pearl, and Platinum games in the same group.
To create a group, players must talk to a man standing in Jubilife City across the street from the Jubilife TV station in Pokémon Diamond, Pearl, and Platinum, or the man in Violet City's Pokémon Academy in Pokémon HeartGold and SoulSilver. After the group leader mixes records, those who have mixed records with the leader can join the group by talking to the same man.
It is possible to leave the current group, and also to create a new group, but it is only possible to be a member of one group at a time.