Was just rereading... I want to edit that's not to say there isn't going to be WAR. Major War. There seems to be some good PvP on the horizon. Seeing as how the three "factions" are all vying for world dominance during this very troubled zombie time there's going to be a whole other side to this game. Of course, since the public beta only allows you to make a character in ONE of these factions, you wont see the tip of the iceburg when it comes to the core conflicts of this game.
No, I don't know how the PvP is going to work yet. It seems like there will be PvP areas. We were told you will be able to PvM (or PvE) and quest with opposing faction. You will be able to party opposing factions. You will not be able to guild opposing factions.
Please note in this game they are not called "factions" or "party" or "guild" I just use these terms because I am familiar with them.