Is your report layout based on the "Agents" table or the Invoices "Table"?
Base your layout on the Invoices table and put fields from the related Agents table in a sub summary part.
Now perform a find on this invoices based layout for the records you want, sort them by agents (use the same field as specified in the "sorted by" field for the sub summary part) and you should be able to produce the report you need.
If you need to list the individual items listed in the invoices and you have a related table of line items, base your report on this table instead of invoices. (You can still specify criteria such as it "paid" status that are stored in the invoices table.)