Instructions
Press the "back" button, the small round button immediately to the left of the "guide" button, to bring up the options menu.
Press the "Right Bumper" button to switch to a list of players online.
Scroll down the list until you find a player you want in your posse. Press the "A" button to select that player and bring up a list of options.
Select the "Propose New Posse" option. This will invite the player to form a posse with you. Repeat these steps until you have invited as many players as you want.