I will be on tonight around 10pm for sure I would like to join a posse!

yeeHa! As far as Rockstar have their people play they did Friday from 4-8pm Eastern. The next event will be Thurs. June 3 from 4-7PM Eastern.
Here some info from the Rockstar website about free roam and forming a posse. You can find from:
http://www.rockstargames.com/reddeadredemption/news
HOW DOES FREE ROAM WORK?
Free Roam is the hub of your online experience. Supporting up to 16 players, Free Roam allows you to explore with your friends and enemies, take out the gangs of outlaws or launch into more structured gameplay using the markers placed around the world. We'll be forming posses and then heading into several game modes during the evening. To form a posse, press the 'Select' button and use the menu. This menu gives you several gameplay options and access to the list of players in your session.
FORMING A POSSE
For PlayStation 3, to start the game, invite your friends, form a posse and find Rockstar, follow these steps:
1. From the main menu, press 'Start' and then enter 'Online Multiplayer' by pressing the 'Square' button.
2. Once in a Free Roam lobby, press the 'Select' button to open the in-game multiplayer menu.
3. Press the 'R1' button and then the 'Triangle' button to 'Show Friends'. Invite at least one friend into your posse using the 'X' button.
4. Once your friends are in the game, press the 'Triangle' button again to return to the 'Players' menu and select them with the 'X' button to 'Propose New Posse' or 'Send Posse Invite'.
5. Once you and your friends are ready to go, start up the game mode we're currently playing. This can be found in the chat as the event rolls along.