Reinforcements are friendly folk who sometimes appear in combat missions to help out. Normally they are beneficial, however if you're on a map with exploding barrels they tend to set fire to everything in sight - including your henchmen.
You can identify the reinforcements as they'll be called "Allies" (usually) and they have a green circle around them (unlike your blue circles and the enemies red circles). You don't have any control over them; they are operated by an AI and make their own decisions during combat.
Reinforcement Types Edit
During the campaigns, you'll often have to work with other gangs to defeat a common enemy. Meetings and rescue mission scenarios will often include groups of allies. In many cases you'll have to make sure specific individuals don't get killed - the easiest way to achieve this is to kill the enemies as quickly as possible!
In Sandbox games, random wet works missions will occasionally include reinforcements. Some will appear immediately, others will turn up near the end of the battle.
Sometimes you get control over when they turn up due to a support role when starting the combat mission - if successful, reinforcements arrive sooner. You can choose not to assign a support role to make sure they turn up later.
Security Agency Edit
The security agency can also provide reinforcements if you've applied the imaginatively named "Reinforcements" upgrade. They seem to join your battles somewhat randomly, although for major events like bank heists, armoury raids and attacking enemy hideouts they seem to appear far more reliably.
Also, unrelated to combat missions, the Security Agency can protect your businesses from enemy drive-by attacks by automatically supplying hired goons; when the enemy attacks the goons will die instead of the business, and the security agency will soon replace the dead goons with fresh non-dead goons.