The reader group(s) for an analysis task are selected from the System/Operator Groups. See screen shots attached.
After you create operators under System, assign them to one of the pre-defined groups (admin, maintainer, viewer) which have rights in the various modules.
You can copy one and modify it to your needs. If the group has rights to the module, in this case NTA, then you can select them and add to the analysis task you created. If they don't have the rights then they would not be in the pick list.