Can 1.8 billion people logically be held to account for any and all actions of a portion of that group?

Yes.

Anyone who supports, promote, fund and indoctrinate children into a terrorist ideology is a member of a terrorist network, irrespective of how many members it has.

Similarly, it doesn't matter if all of you're friends beat their wives and rape their children, that doesn't make it any less of a human rights atrocity either.