Do you have a Web Reporter Premium license? If not, then you are limited to using the Quick View reports.
Under Quick View you can go to Custom and then run a Sites report with a category filter. The tricky part is the category filter. The trick is to add an empty category filter with the checkbox to include the "-" content enabled.
After you run the sites report, you can drill-down to run the users report. Otherwise, you can do the same thing as above for the users report.
You can use the attached report definitions. I also included sample report output. The basic report is just two tables to show which uncategorized sites are hit the most, and the which users hit the most uncategorized sites.
The advanced version will generate the same list of top uncategorized sites, then run a user report for each site.
You can copy the report definitions to .../reporter/docs/reportDefinitions
Then go to Reports > Advanced > Advanced Reports > Import/Export to import them.
That is what my advanced version does using the "Run report multiple times" feature.
That feature can be summarized like this.
1) Make a list of items (can be user names or sites, or categories, etc)
2) Run a report for each item using it as a filter (user filter if item from 1 is a user, site filter if item from 1 is a site, etc.)
So My advanced report makes a list of uncategorized sites sorted by hits descending. Then in step 2, it runs a user + hits report for each site.
Check the example I provided. Let me know if you have questions.
I will give you one warning. This is very much a 2 step process. Most people miss the filter button on the report advanced screen. You need to choose the correct filters to populate your list for step 1. Typically you need to set the date range to match the report. For this example, I also set the "Uncategorized sites" category filter.