@superbattleshipyamato123
An Army Group consists of (at least) two Armies - plus 10 landbased combat units (some of these 10 landbased combat units could also be units from a third Army, since these units would count towards the total of 10 landbased combat units).
An Army Group gets a bonus dice roll with 2 dice at “4” during an attack - and “5” during defense - during each round of combat for as long as the Army Group is intact.
The 2-dice bonus rolls represent the cohesion of the units fighting together within the Army Group.
Within this Army Group, 1 of the Armies also have extra bonus dice rolls - which represents the cohesion of units within that Army.
Remember:
When multiple Armies from the same side or Nation occupy the same territory, only 1 Army can obtaion its ability to fight as an Army - and get the bonus dice rolls.
The units within other Armies in this territory - whether or not these Armies are part of an Army Group - must fight at their individual unit strength.