Dawn of the Dead's third act has brought something to my attention that I'd not considered. Given two groups of humans that meet up, I'd just kinda assumed that they'd coalesce and work together. But now, I realise that's not necessarily the case.
Imagine a group of humans holed up in a highly defensible position, with a fairly large (though finite) supply of vital needs (food, water, etc). Now, another group of humans who have been roaming the countryside, moving from food source to food source as the zombies build up around them, arrive at this stronghold. What happens?
From the point of view of the current inhabitants: you're increasing your number. That means fewer supplies to go around. On the flipside, you'd have more people to help defend in case of a zombie attack - and in the day-to-day running of things, if you're actually holding a functional stronghold such as a farm. Of course, that assumes that the people you let in are actually going to be useful, instead of freeloading off your generosity. Depends on how desperate you are for people, in the end - but unless you're a relatively altruistic collective, you're unlikely to be letting them in purely out of the goodness of your own heart.
From the point of view of the new arrivals: it's pretty similar. If they let you in, it'll be to a situation where supplies are stretched and so forth. You'll also likely be required to fit into the "way of doing things" that would have already been established. The bonus is greater numbers and people who (presumably) already know how the place works, know its strengths and weaknesses, etc.
There's also the "Lord of the Flies" problem. You've got two different systems of control - at the very least, different leaders within each group. How will the control structure in the new group work? Will the leader of either group be prepared to take orders from the other?
Someone also brought up the point about groups of friends in real life all getting into the game together, as often happens with MMO games. While a group of friends may want to play defence together, it's also very likely that they'll play as roaming gangs.
In the end the question as to whether human group encounters will be friendly or hostile becomes quite a toss-up. So I'd better be sure I allow for marauders.
Firstly, drivable vehicles are a MUST. And not just drivable ones; ones which you can load up with supplies and a couple of people, and have someone drive while people point their shotguns out the back. Nothing more effective than plowing a truck into enemy defences and your people pouring out.
Secondly, supplies will have to be fairly dynamic. Things like containers will have to be set up to allow people to carry fuel around (syphoning it off other vehicles, too?), and to allow them to stockpile it. There will have to be mechanisms for transporting everything around.
Also, I've got to consider the motivations of my marauders. Maybe they're just looking for food and water, but maybe not; in the film, the raiders were grabbing everything that is normally deemed 'valuable', things like silver cups and jewellry. Why were they doing that - force of habit, for their own consumption, or trade? Is it possible that human trading posts could spring up, with a market for stolen jewellry and such? In the game, if such items are not usable, then I suspect not. What we might see, though, is a market in ammunition and things like building supplies - wood, nails, hammers, power tools, etc; everything you need to keep a home safe from the zombie hordes. Plus, of course, food and water.