Experienced D&D players quickly learn that it makes sense to concentrate their attacks on one enemy at a time until they are removed from the game, thus reducing the number of attack rolls being made against them. A logical strategy for D&D, but rather "gamey" and lacking in verisimilitude.
Similarly, regardless of the number of men-at-arms, shield mates, and comrades around a 1st-level Wizard who decides to try and help the party out by making a melee attack - a single round of bad luck can kill the character and take one of the players out of the game.
I find the standard game mechanics in classic D&D less than ideal for groups fighting other groups. Whether abstract or with minis they do a better job depicting skirmishers than they do characters fighting in close ranks or otherwise trying to assist and defend their comrades.
Looking at how other games handle fighting in groups, I think there's a way to make D&D combat run a bit more to my liking - without adding much more complexity to the game.
A rule found in many war games that I quite like is that the defending player is the one who gets to choose which members of a damaged unit are removed from the board. I think introducing this system into D&D, Labyrinth Lord, and Swords & Wizardry combat would be fairly straightforward as well.
Player characters can choose to fight in a group with any of their allies (other player characters, henchmen and retainers etc). When any member of that group is "hit" in combat the players can decide which member of the group will lose hit points from the attack. This must be decided before any rolls for damage are made. All players with characters in the fighting group must agree with the reallocated damage roll, or the GM assigns it to the original target.
Players might choose to have damage dealt to their retainers first, possibly resulting in more casualties, but allowing them to continue adventuring longer with their main characters. Alternately a player with higher hit points might want to assign hits against retainers to their PC first, keeping more of the party's swords in action against their enemies.
Since the members in a group are keeping close to one another, all characters must choose to melee attack against the same target - although that target could of course be an enemy group! Additional individuals and groups that choose to attack the players group may also be attacked in melee combat as well. Attack rolls are made against individuals within the group as normal, the only difference is that the defenders may choose to reassign the damage roll to another character
This system allows weak or injured characters to be protected by their comrades, although not without additional risk to the defenders. Fighting in a group means that characters can lose hit points from attacks that would not normally be able to hit their armour class if they choose to shield an ally from harm. Trying to keep an unarmoured wizard or merchant protected amidst a swirling melee is much harder than simply focusing on the fight at hand!
Introducing fighting groups to the game can turn combat into something that requires a bit more decision making from the players. However, the players should be aware that while they can use group tactics, their enemies often will as well...