map will get a primitive array from the Assignees section and email is the raw name for the fields holding email in the Assignees array. It can be different so just hover over the emailAddress fields and you should see the raw value for it.
And, add() function first value will be a list of arrays from the Assignees field and the other one is the Driver email that you are getting from the Custom Fields
After these are set up, flatten will fix the resultant arrays by removing subarrays and merging the result. And, you can use the aggregator and do the same thing that you are currently doing, but iterate over the Set a Variable Module output.