Hi ed_integrity - can I confirm which LMS your submissions are made to? Is it Canvas?

OK thank you ed_integrity .
When a student submits to a group assignment in Canvas, Canvas creates a new pseudo-student called Group User. The Group User IDs are unique, which explains why the Paper ID does not link to comparison files. 
I'm going to investigate if there is a workaround that you can use.

One thing to note: Authorship highlights patterns and exposes differences in the student's usual authoring behaviours and writing patterns. Group assignments are written by multiple students, and so the authoring behaviours and writing patterns will naturally be different to the individual's usual behaviours. Keep this in mind if you are comparing a group submission against files from just one student.

If you are looking to check if the group submission has perhaps been purchased from a ghostwriter, you will need to look at the metadata, such as:

• Author - this is the name that the authoring software is licensed to when the file was created.
• Software - which software was used to create the file? Versions in a language the student does not speak could be a 'smoking gun'.
• Date Created - was the file created before the assignment was set?
• Keywords - are there words on the first two pages that might indicate that a ghostwriter completed the work using a template? Are there keywords remaining from that template that the student should have edited before submitting? "Insert Name of Student" is an obvious example.

Always happy to jump on a Zoom call to discuss examples together - you can reach me via email: mryan@turnitin.com

Ah OK, I see!
In this case, because the work is submitted to a group, the submission is linked to "Group User" rather than the student name and therefore Authorship will not connect this group submission file to the student's previous individual submissions, or any submissions made to other group assignments in this manner.
I can suggest using the File Upload report generation method. While you would need to acquire and download individual files written by the target student, you would then upload their submission to the group assignment as the investigation file and up to 10 comparison files.
A quick way to attain those comparison files would be to run a report using Paper ID method using a Paper ID that you know was submitted to an individual assignment rather than a group assignment. You can then download the resulting  files.