Mostly the same as the Half-Life allies, except there is one extra team on your side...

The Allies
Name Description Picture
The Military Of course, you're the military, so they should be on your side, but don't get too comfortable - unlike in Half-Life, you won't find 20 Military men at once, expect about three. (Remember, you missed the escape helicopter, so most of them have left already.) The Military
Scientists These are very useful for opening security doors and giving you information. Scientists
Security Guards (AKA: Barney or Otis) Security Guards are the only people on your side, other than the military, that have weapons. They can help you through some nasty fixes. They can also open security doors for you, although I thought that would be obvious. The thin versions are sometimes known as Barney, with the fat ones called Otis. Security Guard
The G-Man (AKA: The Man in the Blue Suit) Not much is known about this guy, but you see him quite a lot in the game. He doesn't hurt you, so I suppose he's an ally. The G-Man