'Run' is not a plan. 'Run' is what you do when a plan fails.
In addition to the single player campaign, two new co-op friendly modes have been added. The first, Land Grab, is a Free Roam affair that has players trying to maintain control of certain areas of the open world map for a set period of time. There are seven markers located throughout the world. Simply walk up to one and stake your claim. Be forewarned, you are basically picking a fight with the rest of the players in the session. An announcement goes out to other players that you have staked a claim, and they come after you like you insinuated something about their collective mamas.
This game mode cannot be found in Friendly Free Roam, since the object of the game is to shoot your fellow gamers as they try and jump you claim. You can posse up and work together as a team to hold land against other players. The more players you defend against, the more experience is awarded after the match.
The second co-op game mode is the zombie-flavored Undead Overrun. Two to four players partner up in a lobby, similar to the co-op missions from the free Outlaws to the End DLC. Each game will take place in one of five graveyards found in the world. You can choose from four classes: Ravager, Overkill, Longshot, and Mauler.
The game pits your team against progressive waves of the undead, each harder than the last. A timer is always winding down. Once you are out of time, you enter a sudden death mode where you can no longer be revived by you teammates should you fall. The only way to add time to the clock is to open a coffin which lightning-spawns at the beginning of each round, usually on the opposite side of the map from your team's location. After fighting through the zombies, who are constantly rising from graves, you can open the coffin by pounding the 'Y' button. This will give you another precious minute and ammo for everyone's weapons. Just hope your team has your back.
I was the main coffin-opener-guy for my team for a few sessions, and I'll let you know now there is no glory in it. While you are trying to open the coffin, your teammates are killing zombies. My team was actually very good, so I managed to pop open coffins with ease, but which would you rather be doing? I was adding valuable playtime to the clock, but there is no 'Coffins Opened' stat at the end of the game. Experience is given out equally at the end of each session, so I guess it's just a matter of play style.