As soon as you see an Agent going down his icons on the bottom of the screen will change to this...
(in this case the Agent is already dead but it gives you the idea)
Immediately run up to him (or have your third teammate do it) and when prompted hold
to fix him back up. The Agent will be ready to get back into the fight after the 3 second reviving sequence.
Only on Commander difficulty it works a bit different. Here you will just stabilize him. He will be out of order for the rest of that firefight but come back up immediately after you killed the last alien.
This can also be done in the Hangar 6 R&D dlc.