A player's posse is a group of up to three creatures in the Creature Stage and Spore Galactic Adventures. There is no equivalent in Cell Stage, while in the Tribe and Civilization Stages there are much larger groups (the player's tribe and their motorised forces respectively) that the player has almost complete control over, unlike the posse which simply follows the player around and helps with socialisation and combat.
Creature Stage[]
A posse of 1 can be recruited once the first brain level is reached. Each subsequent level adds another slot, until the maximum of three at the end of the stage. Posse members of different species to the player become pets in the Tribal Stage. Having three posse members of different species leads to the Village Folks Achievement.
Space Stage[]
The posse in the Space Stage is called the "crew" and can be used during Adventures. Similar to the posse is the allied fleet, which is only different in that it is composed of spaceships and not creatures.