In order to obtain this trophy, you must be playing in free roam and counter another player's free roam mission. You actually don't seem to have to make a posse to make it work. You will be notified in game if somebody nearby has started a free roam mission, which you will then have the option to attempt to interfere with. If you can stop the opponent(s) from completing the free roam mission, you will get this trophy and a small reward for your troubles. While there is a chance you will get this through natural progression, it is very easy to boost if needed.

Boosting method by Barnoz:

Simply invite someone you know into your game so that you are both on the same server, but don't make a posse. Have your friend walk to an NPC with a free roaming mission and accept it. Try and find someone in the open world that isn't in a building. He picked someone outside, sitting on a bench near a post office. When he tried doing one where the NPC was inside a house, he got disconnected and switched to another server by the game. Once his mission starts, simply interfere with it so it's impossible to complete. In the case of riding a carriage from point A to B, I killed both horses and the trophy popped. And that's it really. Takes a few minutes at best.