Join Table help

OK, maybe I now finally understand the question. What I would suggest doing is to create a new table called Member Calls.  This would be a Join table between Members and Calls.  It would also be a child of Assignments.  So each record would have three parents. 

Then, for example, when an assignment has been completed, the user would push a button to update a date time field, and that would trigger a pipeline.

The Pipeline would create a Bulk Upsert for the Member Calls Table.

Then it would Search for all of the Calls within the Assignment and then in the For Each Loop, it would add a row to the Bulk Upsert to populate {related Assignment] [Related call]  and [Related Member].

Then the last step would be to Commit the Bulk Upsert.

A Bulk Upsert is the best practice as compared to directly adding those records one by one inside the For Each Loop as it will be less impactful on the performance of the app for your human users.  Also, if you try to add the records, one by one, it is theoretically possible that your app might get over busy and the Pipeline could fail along the way.