I was recently trying to do something similar and couldn't find a way to do it while being able to see see zero values (those unassigned). You could do an assignment report and group it by job role but then you won't be able to see non assignees and you can't reference anything useful from a job role report that I've found.
What I ended up doing was a user report, filtered by job roles and then I was able to add a custom column for workitems that shows tasks they are assigned to and then we can see if there are blanks for users.
displayname=Assignments
listdelimiter=<br>
listmethod=nested(workItems).lists
textmode=true
type=iterate
valuefield=task:name
valueformat=HTML