JOIN your UserT to the current Query and pull in their Name.

Is [ALL USERS Completed Query].[Assigned Investigator] the ID?

All data from all relevant tables (Unit/Center Reviews, Unit/Center Investigations, etc.) is pulled by the All Users Completed Query which is a union query. I will try to join the UserT to the underlying union query for the report.

Alex, this is my complete SQL on the Union query I am trying to get the name of the assigned investigator in...

SELECT [ALL USERS Completed Query].[Case Number], [ALL USERS Completed Query].[Date Received], [ALL USERS Completed Query].[Date Due], [ALL USERS Completed Query].[Date Closed], [ALL USERS Completed Query].Unit, [ALL USERS Completed Query].[Unit/Center Incident Number], [ALL USERS Completed Query].Allegation,[ALL USERS Completed Query].Status,[ALL USERS Completed Query].[Needed Item],[ALL USERS Completed Query].[Date Item Requested],[ALL USERS Completed Query].[Item Requested From],[ALL USERS Completed Query].[Assigned Investigator],[ALL USERS Completed Query].[UserID]
FROM [ALL USERS Completed Query]
WHERE ((([ALL USERS Completed Query].[Date Received]) Between Date() And Date()-6) AND (([ALL USERS Completed Query].[Date Closed]) Between Date() And Date()-6))
UNION SELECT [ALL USERS Pending Query].[Case Number], [ALL USERS Pending Query].[Date Received], [ALL USERS Pending Query].[Date Due], [ALL USERS Pending Query].[Date Closed], [ALL USERS Pending Query].Unit, [ALL USERS Pending Query].[Unit/Center Incident Number], [ALL USERS Pending Query].Allegation,[ALL USERS Pending Query].Status,[ALL USERS Pending Query].[Needed Item],[ALL USERS Pending Query].[Date Item Requested],[ALL USERS Pending Query].[Item Requested From],[ALL USERS Pending Query].[Assigned Investigator],[ALL USERS Pending Query].[UserID]
FROM [ALL USERS Pending Query]
WHERE ((([ALL USERS Pending Query].[Date Received]) Between Date() And Date()-6) AND (([ALL USERS Pending Query].[Date Closed]) Is Null))
ORDER BY [ALL USERS Completed Query].[Assigned Investigator];

This query shows all cases that ALL users have either completed or still have pending which were assigned in the previous week. This is where I am running into the issues with showing the name rather than ID. I'm not sure how to do a join for a union query like this...

Build a second Query using the UNION one as your starter.