Pretty straight foward. The way I did this one was to join a public room and check the player list, I looked for a player list with at least 10 people and no posse. (I did this casually, just playing Free Roam, took me about 5 tries to find the right room.) Then I just sent all the members a posse request, had about 4 right off the bat, after doing two gang hideouts, had the full eight.
Hope that helps some people, you can always do the 8 person party in one public room and do it that way, but I found it much easier to organize strangers in one place, then get some organized friends into one place. Although having some players in a party to start your posse does help, I did it by myself...I'm that cool...