You can't change the canned Open Invoices report because it is hard-coded to subtotal by customer.

However, you can create your own version of the report that you can sort.

To do this, start on the Reports menu and Pick Custom Reports | Transaction Detail.

Then modify the report as follows:

Filters:

- Remove the Date Filter

- Add the Account filter and set it to All accounts receivable

- Add the Detail Level filter and set it to Summary Only

- Add the Paid Status filter and set it to Open

Display | Columns:

- Add the Rep and Aging and Open Balance columns and any other desired columns.

- Remove unwanted columns.

- Set Sort By to Rep

Run the report.  It'll have the same data as the open invoices report, only you can sort it any way you want, on any column you include on the report.