The pool of available responders is built from those that hear the SIC or shout, just like it would be if a PC was requesting aide. This IC response mechanism allows PCs to respond as well.
NPCs will -react- upon arrival. They won't just show up and do nothing. They will (possibly) engage in combat if certain circumstances are met.
Currently this system is beta. I've tested it, but I'm sure there are going to be issues that crop up. So please keep me apprised of any issues via @bug, and anything major via xhelp (if it's over the next few weeks).
I'm really excited about this and the possibilities it opens up for strategy, fighting, and being smart about the way you go about attacking NPCs that might have backup right around the corner. The system is modular so that we can expand the responses as time goes on.
Imagine if the NPC that was dispatched started aiding their allies upon arrival by healing them or stabilizing their wounds or grappling them and dragging them to the doctor if they were dead.
Lots and lots of possibilities.