If your workflow requires you to invoice task timesheets at the staff members individual billable rate, this is managed in the project's quick invoice page.

From the project's quick invoice page, click Invoice Actuals at Staff Billable Rates from the Actions menu.

Billable tasks will then be populated by the rates of the staff who have submitted timesheets, overriding the task rate. Clicking on the expand icon will display the timesheet summary.