A Player Group is a Variable type, that represents a group of up to 16 players. Each player can only be present once, and they are always in order of their player indexes.
The most obvious use of Player Groups is for teams, but they can be used to contain any combination of players that you might find useful.
You are forced to use those from time to time to achieve certain Actions. Note there is, however, a simple function that creates a group of one given player as well.