These folks are pretty cool. They trust Ellie, for the most part, and let her go on the long camping trip that kicks off all the action in the book. They let her take their nice Land Rover as long as she promises to drive and not do drugs or drink. Good deal. Since they disappear after this, though, that's pretty much all we know about them.
The town's dentist is pretty nervous about seeing Lee and Robyn after the invasion. He gives them a little info and advice, but beyond this isn't much help until Lee gets shot—then he reluctantly sews up Lee's leg and teaches Robyn how to take care of him. Lee might have died otherwise, so while Mr. Clement's a minor character, he makes a major contribution to the book. Plus, it's after meeting him that Robyn becomes fascinated by medical stuff.
The soldiers are a mix of young and old, professional and volunteers. Some of them are very eager to shoot, while others are just as scared of battle as the campers. In general, the soldiers take lots of precautions, though they don't spare anyone, not even kids—two of the teens in the Hell gang get shot, and several others get shot at, though not hit.