The main forces are simply referred to as "Grunts", and you will see them patrolling alien structures and captured areas, doing the day-to-day operations. Individually, they are not exeedingly strong, but they have an understanding of battlefield tactics, and won't hesitate to distract and harass you while higher-level aliens set off their own special abilties.
Autopsy Report: Outsider Grunt
Outsiders are roughly humanoid in appearance and physiology, although larger and stronger than the average human. All specimens display signs of repeated physical trauma, particularly bruises and broken bones, suggesting a vigorous regimen of martial training.
Low-ranking troops of the Outsider forces, these “grunts” are equipped with semi-automatic weaponry but little else. They also appear to receive less nutrition and general supplies than their fellow Outsiders, suggesting that they are on the bottom of the hierarchical structure- military, societal, religious, or some combination of the above.
One trait apparently shared by Outsiders of all ranks is a series of neural implants. These devices appear to allow a rudimentary communication between Outsiders, although the extent of this communication is uncertain.