Please may you describe the file structure in details and tell if you can have other structure. Depending on the file structure, the treatment is different.
For instance, if the structure is:
account_id list_ids
recipient1 list1,list2,list3
recipient2 list1,list5
the solution is probably to do it in JS with a foreach loop and splitting list_ids in a JS array with split function then call the nms.recipient.RegisterGroup
The List update activity is intended to update one List (group in case of recipient) at a time. And doing a standard loop in the workflow is not convenient. The List update is useful for mass-update (as in SQL).
But perhaps someone else could suggest another approach for doing a mass-update with such file structure.
If you have many files, each file regarding a list, the file containing the recipient id list, then I consider than using Collector files and loop for List update activity is more adapted.
Well, hope this could help.
Regards
JS