From Halopedia, the Halo wiki
The UNSC Army and its personnel serves as the primary ally in Halo: Reach.
The army troopers function the same as the Marines from previous games, and exactly the same as the Marines in Halo: Reach. They can trade weapons with the player, drive vehicles, and throw grenades. Three to five of them can be "recruited" into the player's team using the Fireteam mechanic. Once recruited, the names of the troopers (who are all named after employees from Bungie and Rooster Teeth) appear along with their rank and call sign. However, while the troopers will follow the player, they cannot be given commands.
Compared to the enemy, the friendly NPCs have a much simpler AI which can lead to friendly fire. They will, however, take cover, jump away from explosions, and flip overturned vehicles. The army troopers are about on par with an Elite Minor in terms of fighting skill. When in force, troopers can hold their own against much larger forces. While they are effective at range, they are generally much weaker than the enemy with melee attacks. Army troopers can appear individually, or in groups as numerous as thirteen (as seen when arming the missile launchers on Exodus).
If two or more troopers are killed by the player in a row, the rest of the allies will attack the player. During this time, their weapons become so powerful that they can kill the player in one shot.
The troopers have new faces and voices from the marines in previous games. There are different variations of the troopers as well as female troopers. These are only aesthetic differences, however, and do not affect gameplay, with the exception that female troopers cannot have their helmets shot off.
Troopers will accept most weapons, but they will not accept Energy Swords and Gravity Hammers. They can appear with any UNSC weapon, but are most commonly equipped with an Assault Rifle or a DMR. They will, however, accept powerful weapons such as Fuel Rod Guns, Rocket Launchers, and Plasma Launchers. The player can use this to take advantage of the fact that NPCs do not run out of ammunition. They also appear to have the ability to guide Rocket Launcher rockets towards all manner of enemy, while the player can only lock on to aircraft.