Complementary Skills

If each member of the group has a slightly different skill, when they work as a group, they're capable of doing things as a group that would be impossible if they worked as individuals. After awhile, you'll come to think of the group as a single organism.

This technique can be taken to a higher level if the NPCs are programmed to act as a group in a choreographed way. For instance, one guy in your team runs out of ammo, and without missing a beat, one of the others in the group throws him a clip.

This could be done by way of a scripted sequence, triggered by some event in the battle.