A posse is group of people who help the police force when necessary. In a classic Western movie, when the sheriff gathers together a posse, its members usually work to find and arrest the bad guys.
The classic image of a posse is from the Old West, of a group of armed cowboys on horses, in pursuit of an outlaw. Originally the term was posse comitatus, Latin meaning the force of the country. Today, the word posse is used most often to refer to a group of friends or people with some common interest, in a somewhat joking way, like your posse that gets together to hit all the garage sales together.