If the SHTF, a lot of people who formerly were getting by just fine, won't have one single ability to cope. They'll be in denial and without skills or items to trade that will help improve their position, or security, or feed themselves, or find water. Some will band together with other people. Probably one of those hastily formed tribe members, will most likely be better at leading, or at least have some ideas. Maybe they know a few common things to do to survive. If so, as long as things are conflict-free, that new tribe will stick together.
Little things become big things when you run out of caffeine and have a headache. You run out of cigarettes and you need to relax. You haven't had a drink of alcohol in a while and you've been regularly consuming it before the disaster. You're hungry and feel weak.
Stressed out people tend to express their frustration by arguing. It's a way of venting. Of course it seldom solves anything, and only tends to infect the group and make things worse.
Of course, practically anyone knows who's going to “pop” first from the strain. The weakest link will lose it first, then they tend to keep losing it. You can pre-treat it, or you can end up dealing with it over and over.
Most people attempt to deal with the loudmouth first. It probably won't work. It's you versus them, and unless they need you, they aren't going to listen. If you let them vent more than once, then they'll keep on, and you'll start to have terrible conflict in that group.
If they're with someone and came together before they joined with your group, then that person has dealt with them longer. They know them better, if only a little. They're more likely to be persuasive then you.
You can try peer pressure by getting the other group members to reject the new person plus who they brought, and that might work, but in the early days of a SHTF situation, people will think it's temporary and will just “grin and bear it”.
Reason works only with reasonable people. Yes you can talk to them about “not losing it in front of the children” or “you're making everyone miserable including yourself” or “let's go collect some more firewood or food instead” the latter being the most effective means, because you get a little more work done, remove the conflict from the room and possibly talk to them one-on-one. This is probably effective at first with rational people. Then quietly saying something like, “Man I feel stressed and barely holding on myself. I'm trying to come up with ways to cope and I'm running out of ideas. What do you think would help?” Many times a rational person is frustrated and merely wants to share their ideas because they know they're intelligent. Chances are if they have no survival skills, their ideas are impractical and won't really help, but it might help defuse the situation.
Many people need group attention, and they'd rather have bad attention than no attention. Most people want to be liked, so find some way to get them attention in a positive way.
Many times people who are physically strong need to blow off some steam. Chopping firewood always needs to be done and wears people out. It also helps stressed out people to sleep. It also means the group is warmer and probably appreciative of their efforts.
A lot of conflict is the result of long ragged history with other tribe members. They may be family or lovers. In the best of times, they fought, and now in the worst of times, that's all they have left. Those kinds of conflict in a SHTF scenario might lead to murder, quite frankly. Separating them and putting them on different work crews and people individually listening to them vent, let's off some of that pressure from the both of them. Watch those people's tense body language. Their chests are tight and faces contorted with rigid jaws. They speak tersely. Really, what they need is to relax somehow. A joke, a spare cigarette, even a mouthful of food, might placate them and help them control themselves. Mostly they want validation for their viewpoint. Once even a little attention is given, then a little calm lowered voice and resonance by sharing times you felt the same way, and then once the similarity is acknowledged, then the smallest bit of advice will penetrate.
Even though you're all busy, a leader or shaman will often do a little more to help someone do their chores and therefore gain 30 minutes of extra sleep for them. You save that “favor” for another time when you need them to help someone else. You ask them, “Say Henry, I woke up the other day and started the fire for you. Remember? Sally's sick. Real sick. Can you do it for her?” Chances are, he'll feel like the hero, and Sally will thank him. You can't do it all, so pass it around. This is a very good way to lead and builds on itself. Some people will volunteer, but quietly tell them to give other people a chance to do it, because a regular volunteer will become overwhelmed and will keep volunteering and actually screw up the group dynamics.
If one person is argumentative, it might be helpful if you teach them to do some critical skill and so gain praise. Getting praise is way more fulfilling than venting and complaining. Since a low skills tribe needs to gain skills anyway, then it helps both issues.
If someone is argumentative and they're with you in a foraging party, then allowing them to be heroic and sharing the news, “Yeah, we found plantain to heal some of these wounds.” well of course that will work very well towards calming things down and cost you nothing, for the tribe will know who really found it, won't they?
Some loud people tend to get heard in group settings, not because they're persuasive or logical, they just have more volume. This shuts down the quiet person who might have great ideas. You never bring a new idea to the group to vote on. It's not a democracy. You talk quietly with the reasonable people, build consensus individually, then bring it to the group at the end of the day when people are tired and relaxing and so less likely for protracted arguing. Then people will usually agree and know what their tasks are for the new day. A morning meeting wastes daylight.
Don't mistake a challenging person for an argumentative person. A challenging person may see flaws in your plan. You don't want “yes-men” who agree with everything you say. They might save the group. Listen to them, but don't allow conflict to go on if the group decided to do it one way, or if it's dangerous or risky to do it their way.
Tribes will high conflict end up disbanding and dying from lack of security.