If your Army Faction is AGENTS OF THE IMPERIUM, then in the Select Detachment Rules step, you can select one of the available Detachments from this publication as normal.
If your Army Faction is not AGENTS OF THE IMPERIUM, but every model in your army has the IMPERIUM keyword, you can include AGENTS OF THE IMPERIUM units in your army even if they do not have the Faction keyword you selected in the Select Army Faction step. In this case, the maximum number of AGENTS OF THE IMPERIUM units you can include in your army depends on the battle size, as shown below.
In an Incusion sized battle, you can include up to 1 Retinue unit, 1 Character unit and 1 Requisitioned unit. In a Strike Force sized battle, you can include up to 2 Retinue units, 2 Character units and 2 Requisitioned units. In an Onslaught sized battle, you can include up to 3 Retinue units, 3 Character units and 3 Requisitioned units.
Note that you can include AGENTS OF THE IMPERIUM DEDICATED TRANSPORT units in such an army as normal, but each unit must start the battle with one or more units embarked within it, or it cannot be deployed for that battle and will count as having been destroyed during the first battle round.