If a zombie charges the group of 5, the first thing that will happen is a "bystander effect". None of the 5 humans want to waste their ammo shooting the zed when they can let one of the other 4 people shoot first. Essentially, it becomes a game of Chicken. As a result, zeds can often get closer then normal to the group before anyone shoots. Once that zombie gets too close, everyone realizes that they need to shoot or they're in danger of getting tagged. This results in about 2-4 players shooting the zombie, wasting ammo and time that could've been spent shooting another zombie. Applying this principle to a rush of a much higher count zombies on that group of 5 people and I think it's easy to see how more players would die than if the group of 5 were by themselves from the main group. Now imagine the 3 or 4 people behind that group of 5. They're even less efficient than the first 5 people because they have even less motivation to shoot zeds (they'll be safe behind the front line) and are farther away with each shot (I'd put their efficiency at about 20-35% of being by themselves). If all of these people were split up into small groups, the human side in general becomes more effective, as individual players are more efficient at killing zeds. This may yield some groups going down to massive rushes, but I think that the human side overall will have fewer deaths than going as a big turtle group.
There are multiple simple solutions to this but I'd proffer my personal experience (though you'd probably want to listen to those with greater wisdom than mine). My game recommends that veterans spread themselves amongst people if groups form. These are the heavily tooled players who know how to use their blasters and they're usually carrying a lot more ammo than the less experienced players around them. They'll also have the larger and more effective blasters and know how to use them. One of our players, nick named 'Sarge' will always remind people to call their shots in a very loud voice on multiple occasions during a horde encounter - a couple of others do it too but he's the main guy. Usually, someone will shout their target and shortly after will open fire. "Female, blue shirt" "Male beanie hat" that sort of thing, it gets weird when there's a large group of you doing it. No one else then fires at that target and it raises the efficiency of the larger group (commonly around 20 players in a room the size of a sports hall, BUZAN play indoors). Often our veterans are able to lead by example and be reasonably altruistic in their fire but leave enough for the lesser experienced players to get involved - besides, they don't always hit and then the player next to them usually steps in. Besides the vets yelling to call your shots, it's an unspoken system - once people see it's the done thing they tend to join in. You only need to hear the guys immediately around you because, as you pointed out, only 4-5 people share the same lane of fire.
My preferred method of engagement is with a small squad in open areas doing all the things Dyslexda and Chev Mal Fet suggested earlier but the key to HvZ on both sides is flexibility, knowing what to do when a situation changes from the ideal. Getting it back to the ideal is how the humans win (or zombies if they're the ones in question). Asymmetric warfare on the open field is the best idea for humans but sometimes you just have to grin and bear it when people group up.
tl;dr Open terrain all the way for humans, closed for zombies