What is stopping you from updating some sort of data storage location in each process an employee is completing? You cannot easily query the forms database for completed instances for a process and identify the users that submitted them.

When I have built these types of processes in the past I've gone in both directions here:

1. Employees start a process on demand via submitting a starting form
   1. Each instance updates a database or creates an entry in the repository upon completion
   2. A manager/admin has a starting form that searches the repository or queries the database and grabs submissions by employee id
   3. It is then up to you how you want to display the cross-reference here
2. Manager/admin submits a starting form that invokes a workflow that starts a process for each employee in a database table
   1. Each employee submits and finishes the tasks assigned to them which updates a database
   2. Manager/admin can monitor completion directly in forms monitoring/reports
   3. Process in which the manager/admin originally submitted has a new task that searches the repo or queries the database by submission/employee id
   4. Same as 1.3 up to you how to display this data
   5. Parent process is submitted to show final completion