An update for this is now merged here:
https://github.com/discourse/discourse-data-explorer/pull/369
There is now a checkbox in the automation script to send individual PMs to group members. When checked, each group will have the users extracted and message individually.
Hopefully this makes the script work better for your needs.
If adding a group to the Send to User, Group or Email field then that group would need to have access to the data explorer report. The ideal scenario would be to have a larger group (maybe a reporting group) that contains all the members that you want to notify.