The alternative combat system in Men & Magic has few details, so most of us OD&D GMs assume that players pick a target, make an attack roll, and (if successful) roll for damage to that target. And that's the way many of us handled combat in AD&D, too, when we started playing a thousand weird amalgams of AD&D and other D&D versions. But AD&D says something different about attacks:
Who Attacks Whom:
As with missile fire, it is generally not possible to select a specific opponent in a mass melee. If this is the case, simply use some random number generation to find out which attacks are upon which opponents, remembering that only a certain number of attacks can usually be made upon one opponent.(DMG, p. 71)
This is important because the Delving Deeper errata that has just been released adds a new fighter ability:
Starting at 4th level [the fighter] adds +1 to morale rolls of any troops he leads in combat, and is no longer subject to randomized missile or melee hits while there are other targets available.(DD Adventurer's Handbook, p. 10)
Heroes basically can't be harmed by random combat effects; those who are less than heroic are targetted first, unless an attacker is able to specifically aim at the hero. This implies that in at least some cases, melee targets are randomized, just like AD&D.
I think a simple way to handle these situations is to roll damage for the attack, then have all possible random targets roll a d6; whichever die roll matches the damage roll is struck. Ties go to the quickest.