It works like this:
* Humans can get a "heal gun" from the armory. It costs nothing.
* When a human with a heal gun points his gun at nearby humans, they get slowly healed (with some graphics), and he slowly gains credits.
* The amount of healing/crediting is proportional to the proximity to aliens (the proximity should be displayed to the healer guy somehow, and perhaps to people around him - a sound of some sort perhaps).
This encourages teamwork - by giving humans a good reason to keep together - to protect the "healer guy" - and the "healer guy" has a reason to seek out other humans to heal. It means low credit noobs have something to do when they would otherwise have to camp for credits. By making it only effective near to aliens, it encourages fighting.