The Action Group action type has no specific functionality, but is extremely useful for structuring your project.
There are a couple of ways to add child actions to an Action Group (or any other action for that matter):
The Action Group's BeforeAction, AfterAction, and OnStatusMessage Async Action Group, which runs its child actions in parallel.
Any action can be used as a parent action for other actions - you don't have to use Action Group
Action Groups allow you to declare Local variables are only available to child actions of the groups. Since action groups can be nested, you can also override variables in child groups, and actions in child groups can also reference actions in the parent groups. Local variables are also very useful when running actions in parallel.