Another nice feature of pivot tables is that you can group results.
Do the exercise using the exercise file of Pivot Table Groups if you like. The result file is Pivot Table Groups Result.
A common thing to group is the date. Here is the file we start with.

To start the grouping process, right click on an invoice date, then select Group and Show Detail, then Group.

Next comes up the grouping dialog box, which senses that we have a date field and offers a range of data options.

Highlight months and click OK and then we get the monthly summary below

Now that we have set up this field as a group, we can also use it as a group page field. By moving the invoice field to the page field area at the top and adding the rep field into the report, we now have the following version.

Grouping is another feature in pivot tables that you can use to keep the size of your database list down. Rather than creating a separate field for the month, you can just do a grouping on the invoice date and get monthly reports.
You can also do other groupings, such as dollar amounts. Excel will let you set how much of an interval you want- what ever number you choose- to group by each thousand you would choose 1,000 for example.
Groups can be a very quick to summarize your data and get further insights into your data. Give it a try.