‘Posse’ started out as a technical term in law, part of the term ‘posse comitatus,’ which in Medieval Latin meant ‘power of the county.’
As such, it referred to a group of citizens summoned by a sheriff to preserve the public peace as allowed for by law. ‘Preserving the public peace’ so often meant hunting down a supposed criminal that ‘posse’ eventually came to mean any group organized to make a search or embark on a mission.
In even broader use it can refer to any group, period. Sometimes nowadays that group is a gang or a rock band but it can as easily be any bunch of politicians, models, architects, tourists, children, or what have you, acting in concert.