Since its almost been completley confirmed that a Zombies mode is coming to GTA 5, the only way i truly see it working is this:
Please note that the grouping part would only be possible in a seperate multiplayer lobby. Also note that I did post this idea in the rumors and speculations thread but not alot of people saw it.
Day 1: The day starts out normally, people talking, fat guys waddling, the average day in Los Santos. Over the course of day 1, random car crashes will occur. People will get bit and turn into a zombie. No matter how you die you become a zombie. The army and police try to kill the zombies, and the army sets up tanks and military personnel all over the Los Santos area. Zombies will attack cops, pedestrians, paramedics, and of course the player. By the end of the first day, their will be about 500 zombies roaming the streets of Los Santos with 75% of emergency services that remain intact.
Day 2. The world is chaos. People are panicking, and more people die by the minute. You can now loot stores like ammunation for guns and supplies because thes stores will run out of supplies. (people might hate this part but you would need to get gas for your car ONLY in the zombies lobby). You must stock up on supplies and if Zombies is released to multiplayer you can band together with your friends to fight the zombies. By the end of Day 2, the Army is completley overun and so are the cops. Los Santos has fallen and large hordes travel throughout the streets of Los Santos.
Day 3. On day 3, Their are no more wanted levels. Since the cops and army have fallen. Now you can hole yourself up in the prison (yes like the walking dead) or the military base. But please note that if enough of the zombies pack themselves against the fences, they can eventually make the fence fall and the zombies will overrun the prison or army base. Also, their are not alot of supplies in Blaine County and Paleto bay meaning you have to back into the major hordes of zombies and kill them to get supplies like gas, ammo, and food. By the end of the day, Most of the general population has been killed off. The army is overrun, and the government has fallen.
After Day 3, the world remains hell. It is up to you and maybe your group of surviors to survive as many days as you can. Good Luck
(I accept hate on the idea but please dont be all wow move this this thread shouldn't be here)