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.

When voice chat is enabled, the player can hear their fellow play-mate from the other side of the map - as mentioned in a video (Multiplayer free roam)

In a Posse a player is able to do all of the mini-games available in the single player, This includes Arm Wrestling, Poker and Drinking

Players can also go hunting together in specially appointed areas to fend off waves of animals. Apart from hunting, players can also track down Plants and conquer gang hideouts together.

Posse Leader
The Posse Leader has control of certain things that govern what a posse is doing. For instance, the posse leader is able to queue the posse up for competitive games, and set waypoints for the posse to go to. Other players in the posse can also automatically teleport to the posse leader's position from anywhere on the map.

Achievements
Forming a complete posse will gain the player this achievement:
 * Posse Up!: Create a posse with the maximum amount of players.