Whilst in a posse in free roam you occasionally will see a message pop up on screen letting you know that a rival posse has stolen a wagon/money bags or something of the sort. When you see this message go and intercept said scenario, kill the rival members if you need to and most importantly steal the money bags or wagon and deliver them to the location on your map, once they are delivered all members of your posse will unlock the achievement.
If you so wish you can boost this with ONE other player. Both players form a posse and have turns starting a free roam mission whereby you intercept a wagon and steal the money bags/wagon or any mission that offers kidnaps, mail bags, Wagon recovery or horse thefts. Simply have the person going for the achievement waiting nearby and have them swoop in and intercept the mission as soon as the message pops up saying you can rival a posses mission.
The mission just east of Armadillo at MACFARLANE`S RANCH with Bonnie is ideal to boost this, or any mission that offers kidnaps, mail bags, Wagon recovery or horse thefts.
PLEASE NOTE: The mission with the canoes does not work for this achievement nor do the hunting missions.
Alternatively you can attain this achievement if you are targeted as an assassination and you escape.