You can join a posse that has already started earning points in a public free roam and the points will still help you earn the trophy. You can also form a posse of 8, but you don't have to earn points together, just while in a posse. For example, half the people can split from the group of 8 and do Hunting Grounds while the other half can go and do Gang Hideouts. Just stay in a posse and don't break it. Hunting Grounds are black circles with 2 white circles inside and Gang Hideouts are black circles with a white bandit face and hat.
If your teammate leaves or gets disconnected, your posse’s points will drop and you will only be left with just the points the remaining player(s) have. When the disconnected player joins again he will start off with 0 points.
If you get 50,000 points and you invite other players to your posse they will get that trophy too, as long as they have the DLC and you still have total posse points of at least 50,000.
To do this with 2-3 people, form a posse and go to Solomon’s Folly where you can net about 500 or more posse points in about a minute. This trophy will take around an hour and a half to obtain with 2-3 people.
A better method if you have a bigger posse, 5-8 people. The trophy will be obtained in less than an hour if you achieve more posse points. The narrator will alert your posse to your progress. He will announce when you reach 1,000, 5,000, 10,000 and then after you reach 50,000. He even announces your individual points.
There are two ways to check your stats: while in free roam right after the Hideout you are doing has ended and the stats page pops up, and when you are not in a mission.
1. When you finish a Gang Hideout, hold and the points will pop up with your score and the players in your posse; next to the posse leader will be the total posse points.
2. After you complete a Gang Hideout or Hunting Ground, you get the personal stat screen that says Total XP, and press to get the posse points for that run, that is your score toward your posse points + what your posse gets.