I love "guarded by" triggers. They are so easy to use and so useful. Basically, whenever some enemies are spotted/engaged near a "guarded by" trigger, than nearby friendly groups on a "guard" waypoint will move to engage the enemies near the trigger. Note that this doesn't happen until the enemy is spotted, unlike what AK-Chester is claiming.
I'm not sure about the specifics, such as how close a group must be to support a trigger, or how many groups will support a trigger at one time. So if you have lots of "guarded by" triggers, and lots of "guard" waypoints, spread far across the map, you could run into undesired results (i.e. squads moving all the way across the island to support). This is why it is somewhat of a "blunt" solution, as macguba said.
If you do have a situation like above (lots of enemies/guard locations spread over a wide area), and you want to have very precise control over which groups can support whom, then I would suggest using a script such as Bremmer's AI script or Keycat's grouplink script. But if you have something smaller or simpler, I would highly suggest using guard waypoints/triggers, because they are extremely easy to use, and work better than those scripts at what they do.