So far I can only mirror single objects. But how could I mirror a whole group of objects? I can’t find a way to do it.
You need to be more specific on your exact requirement.
Do you just want to mirror something (Ctrl+M to selected objects) or use a mirror modifier (Only applies on an object by object basis but can copy the modifier to multiple other objects).
You cannot apply a mirror modifier to a group so you need to achieve the same result another way and this is purely dependant on your scene and what you exactly want to achieve and what you want to do with this result.