Because you can select all you agents in one go there's a temptation to just move them all together and let them slug it out with the approaching enemy. However, keeping them in a block like this is not tactically sound. Firstly, it makes them an easy target for an area of effect weapon such as the Guass gun. Secondly, it's not using them to your best advantage.
Deployment: spread out your agents for optimum firing ability
If you keep them in a block, one or two agents will be closest to the enemy, so they'll be the ones doing all the firing - they'll run out of ammo before your other agents have done much firing at all!
Also, because the enemy often get pushed back out of range by your weapons fire, they'll hang around for a while before trying to get into firing range again. This means that your agents will be getting tired and addicted to their drugs, which makes them a less effecient fighting force; a situation that the enemy might be able to take advantage of.
The best way to use your force is to move them one at a time - your people can't fire on the move, so even if you only have two agents then keep one stationary so that it can fire, and move the other forward just a little. Then, when the moving agent stops, bring the other to meet it and then on a little closer to the enemy. This means that the enemy can not take advantage of a break in your firing.
Early in the game this can be used to take better advantage of weapons that have more firepower but lower range. If you have only one agent with a working Uzi (perhaps because the others have run out, or because you've only been able to capture a single gun), keep this agent firing at the enemy. The Uzi has range but not much punch. Another agent can then creep closer and use a shotgun - which has very little range but much more stopping power. Because the enemy have been kept busy with the Uzi they won't get much of a chance to fire on the agent with the shotgun. If you'd have just taken in a force of shotgun-wielding agents you'd have probably been taken out before you got close enough to fire. It should be noted that the Police often have shotguns, and if you kill the police their weapons are always full.