Follow the steps below to generate a report of all contacts related to a category:
1) Click new, create a report name and click next
2) Delete both conditions and click add relation
3) In the first drop down field select “related to”
4) Second drop down field “categories”
5) Choose the category in the third drop down field and click next
6) In the “available fields” column shift company over to the “selected fields”- this will give you a list of all contacts and companies related to your category.