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Users added in QuickBooks Online (QBO) will be able to use the mileage app since QBO is a cloud-based program that can be accessed anytime and anywhere. You'll want to add the user to QBO.
To track mileage, you'll have to turn it on. Please see this article for more information: Track Mileage in QuickBooks Online.
Feel free to let me know if you need anything else. I'll make sure to help.