This may not seem to be important, but I believe that it's vital, and something that you might not think of in group dynamics. Since a lot of people are imagining surviving by themselves, and not thinking about the entire tribe or community, I hope you temporarily suspend disbelief and consider my words.
Imagine a group walking through the woods. Because our nature is to make trails, these snake back and forth in patterns based upon the ease of constructing a path. Since the trail is narrow, the group thins out. The party of people are all looking at different things they are momentarily interested in: what catches their eye, what they suddenly hear, what they are picking up and touching, what smell is wafting on the breeze, etc.
When the trail is flat, the group tends to cluster together, share stories, and people listen to each others. Still there are lots of side conversations because people like several in the group for some special attribute, and pay more attention to those people.
As a trail ascends, and because trails are usually narrow, then the group gets tunnel vision, and often greenhorns will look immediately to their front and what they are dealing with as they huff and puff with the exertions. Stronger party members, with better constitutions and more physical strength and stamina, will move quicker and sometimes pay less attention to any details. Weaker ones will straggle and slowly move behind. Over time, the group thins out and elongates. Sometimes it becomes more like a race to finish instead of a journey to travel together and experience.
As a trail descends into a valley, the group's distance between each other will shorten, and the group gets closer. The vista will awaken their awareness. The picturesque scenery will make them aware that they've been missing details and they'll renew primarily their visual input as they see something beautiful individually like a flying bird, or hear the distinctive tap of a piliated woodpecker, or the breeze brings fragrant Basswood flowers into their natural senses. Observations are made, the group becomes one again.
Or not, because some groups become more focused on navigating down the hill as some member is struggling to climb down a steep decline, and so it still thins out due to the steepness and each step into the soft earth creates a dangerous chance to pitch forward, and because one can fall, another might fall in the same spot, so people watch to see if that person's struggle may affect them when they get to that loose soil and the big steps downward. They note where that person grabbed on, and so they might benefit from how they navigated, and either employ the same technique for the descent, or do something entirely different.
When this happens, the natural response is not to help the struggling person, because most people don't descend as a group, they do it individually, and so they don't reach out and use each other's strength. Maybe they don't know what to do, so they are silent. Some more altruistic people will help, or offer guidance, and so with minimal assistance, the person in that loose soil will benefit for an instant from helping hands or words.
When a group walks together however on flat terrain, then it develops mass. It's an array of people most often made of leaders in the the front and back and sides. In the center is a group of less strong people with less individual skills. Their mass makes them a force because of their momentum and velocity. It's why a football team or a group of soldiers, can overpower an opponent. If the formation has weak areas, or a lack of discipline, in that pocket will for a gap and that will be where the formation will break under an assault.
There are reasons not to be in close formation. If the group is under ranged weapon attack, if close together, then many more will fall then if spread out a little. If the group is ascending, or moving rapidly, it tends to form a triangle and elongates again, but it doesn't have the same power. If the strong one are fixed and resolute, and the weaker ones close ranks and work in unison with the leaders, then that wedge formation will penetrate the opposition's own weak areas. Then the opposition will break and usually a rout occurs with them losing heart and scattering. In a descent, it is easier to keep a tight formation as one group.
Let's imagine several groups. If the group are made into a line, then one side clashes with another opposing line, then the two will butt heads, and the one with better discipline and order and resolve and strength will finally prevail, but at high losses. It's a head on formation and someone will eventually win out, but the attrition can be enormous. What usually happens is a series of individual battles instead of a group fighting a group.
If instead the group is weighted such that three strong squads are three deep and the rest of the line is resolute in their single squad thickness, then the mass of the three deep when it hits the enemy line, those shock troops will break the enemy line. It looks like an inverted L shape and the second and third squads added mass will overrun the gaps in the enemy's resolve.
If the faster and more dexterous squad members can come from the sides and then get behind the enemy, they can flank them. If the main group stays resolute, then the additional flank will overpower the enemy, because they can't deal with two attacking groups at once. It's strongest when the attack comes from front and back, but if not timed as well, the attack from the front and side will also overpower the enemy. They pin the opponent by this formation.
All of these formations mean coordinated attacks and defense, because the movements are not static, but shifting and constantly dynamic. Which ever ones can stay alert, focused, disciplined and follow commands will usually win. Interestingly, the one who wins is the group that has better morale. That group will usually have more cohesive power, and the enemy senses it and falters.