When members of a group take heroic actions to protect each other, group bonding increases. These are physical actions taken by the NPCs in your group. For instance, in a battle, they station themselves over a wounded comrade and fight to save him. If you're their leader, they may try and protect you at all costs.
This becomes a bit richer experience if the characters are a bit more dimensional. For instance, the NPCs could sometimes complain about each other, but then, in battle, their true loyalties could comes out. This was done wonderfully in the original Star Trek series, where Spock and Bones complained about each other and bickered, but didn't think twice about putting their lives at risk to protect each other. (It's also a classic example of NPC to NPC Relationship Deepening, which is discussed in Chapter 2.8.) Similarly, Captain Kirk wouldn't hesitate to put his life on the line if he thought it could save his crewmates.