Posse

A Posse is a group of armed individuals, with a common goal. In the American west, Posses were formed by lawmen, and were composed of able-bodied men, deputized to assist in enforcement of the law, or in the pursuit of outlaws.

In Red Dead Redemption, a posse can be formed while playing online with other players as part of the Free Roam mode. Up to 8 players can participate in a Posse.

Achievements
Forming a complete posse will gain the player this achievement:
 * Posse Up!: