I thought you knew, according to your screenshot, since it is the correct way to create your component. Then, to initialize, you just have to set any property from here :
SpellCaster->SpellArray.Add(whatEver));
Or set the values directly in the constructor of your UActorComponent.
About the RootComponent, I did not see that it was an UActorComponent and not an USceneComponent. And as you said in your previous comment, only USceneComponents can be attached. So, you should inherit from USceneComponent instead of UActorComponent.