What is a group of troops called?

It depends on how many there are. It can be called a squad (10- 20 Soldiers), a platoon (21- 50 soldiers) an so on.

In the US Army a company is a formation of 100 to 150 soldiers. Companies may be divided into four to six platoons; each platoon is then divided into squads or sections that may then be divided into teams. Detachments are units smaller than a normal company.

Field artillery companies are called batteries and cavalry companies are called troops unless they are mechanized infantry.

Several companies form a battalion; cavalry battalions may be called a squadron. Several battalions may form a regiment, but this may be ceremonial with the battalions scattered across different commands. Battalions are more often formed into brigades which are then formed into a division.