What does "Posse Up" do?

[MissyXS]

Simple question that I would like confirmed:  What is the difference between selecting "Posse Up" and "Free Roam" on the main menu?  At first I thought there were 2 different types of servers: one where players were all thrown into random posses, and another where people spawned on their own and could create posses if they wished.  If a temporary "open" posse was created in Free Roam, I thought only players on the same server would be able to join, probably from the D-pad menu.  However, I am now convinced that there is actually only one kind of server: Free Roam, and if you select "Posse Up" from the main menu, you are auto-joined into a random temporary open posse that someone already created in Free Roam. 

For any newbies Googling this topic, please confirm that I am right?

[Kean_1]

There is only one Free Roam at the moment and there isn't a specific posse session the game will place you in.  While "Free Roam" inserts you into the game alone, "Posse Up" simply adds you to an open posse in a session.  You are in a game with others that came in alone, as a "Posse Up" option, those who joined with friends, etc. 

[riptide87]

It is basically a quick entry feature to join other players you may not know so you can do tasks and missions as a group if you need to. You aren't obligated to stay once you join either. 

[virgil_caine]

It gets you shot mostly...