Addressing your second question since your first appears to be solved. Create 2 reports:
- An assignment report to be used to test your solution report
- Your solution, which is a user report.
In the user report filter, put in the following in text mode.
EXISTS:A:$$EXISTSMOD=NOTEXISTS
EXISTS:A:$$OBJCODE=ASSGN
EXISTS:A:assignedToID=FIELD:ID
If a name shows up, you can test it in your assignment report. E.g. the name of John Smith is listed, so in your assignment report, filter on Assigned To ID = John Smith, and the result should be no assignments, thereby proving your user filter (nothing assigned) works.
If I can make a suggestion - this is probably an incomplete question on your part so maybe think over what you really need and restate the question. The filter I provided pulls in everyone with exactly zero assignments and it’s usually better to pull in people with zero task assignments or zero issue assignments, or people with zero assignments on active projects, or people with zero assignments on tasks that are incomplete -- these are just examples that I’m listing -- not a comprehensive list.