Currently there are no reporting features within Access Expense and a development note has been made to get this added for future releases.
You can however use the All Claims export feature to achieve this.
As an admin complete the following:
Click History, All Claims, Export
Open the exported .csv / .xlsx file in Excel
Filter the list using Expense Type.
Delete all expenses that were not mileage claims and use a method to list which users raised them.
The list can then be filtered again to calculate how many miles each employee claimed for.