When Reese tells Sarah the story about "his world", the door to the bunker is manned. Then after we see Reese with Sarah's picture, we hear the dogs barking and then all of a sudden the T-800 gets through with two hostages. How does it get the password that Kyle Reese had to tell at the door?
Its a dream. Dreams don't have to make sense. Though this one does, you just seem to be misinterpreting it.
There were no hostages. It was doing the whole point of Terminator, blending in to infiltrate. It was following humans who had no idea what it was. It was the dogs barking that revealed it to be a Terminator. The Terminator then takes out its gun and kills the guards before they have a chance to fight back.
The Terminator was part of a group of "unregistered scavengers" that had been found in a nearby shopping centre and invited into the base. It was waiting outside the base in an entryway and pushed through the guardpost when the door was open (rather than waiting for its screening, knowing that the dogs would have detected it). This gave it the slight advantage of surprise.
Reese glanced up as he heard the security door being unlocked and opened. Another patrol was coming through, visible in the distance under the dim portable fluorescents.
Two scouts. The dogs were smelling their hands. Then another man pushed through before the sentries could close the door.
He was a head taller than the others and carried something bulky under his torn gray poncho.