From Bulbapedia, the community-driven Pokémon encyclopedia.
The Chimecho Assembly is a system run by Chimecho in Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness and Pokémon Mystery Dungeon: Explorers of Sky as a replacement for Friend Areas from Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team.
The player can access the Chimecho Assembly at a desk in sublevel one of the Wigglytuff Guild, or by ringing the bell in the well at the crossroads south of the guild. Here, one can arrange which Pokémon can be chosen to go out on expeditions to various dungeons. However, this feature can only be accessed after Chimecho calls the team to the location of the Chimecho Assembly and rings the bell. The limit of an exploration team is four members, meaning before the player and their partner graduates, only two Pokémon can be chosen from Chimecho Assembly.
After the graduation exam, Chimecho will ring the bell again, this time allowing the player to choose which Pokémon will be the leader in exploring a dungeon and also allowing the main player and their partner to not be on the exploration team. This is a helpful feature if the player goes to a dungeon in which the player or partner suffers from type disadvantage. Unlike the previous games, even if the main player and their partner aren't on the team, they will still be the ones who go around Treasure Town, and not the new team leader.