To unlock the Posse Up trophy in Red Dead Online, you need to form a Persistent Posse as opposed to a Temporary one.
In the game anyone can posse up for free by going into the left d-pad menu and selecting Posse. From there a Posse can either be joined or created. The Temporary Posse only lasts for the duration of the session (which, like GTA Online, means that it's lost if you go into a mission and then revert back to Free Roam.
For this trophy you need a Persistent Posse. You'll have to save up $200 for this, which isn't too hard if you hit a few main missions, sell a bunch of meat to the butcher and don't spend your money on anything else. Go into that same d-pad menu and create a Posse - make sure to select Permanent. Name it, set up the rules and create it. The trophy will pop immediately even if you are the only member.