You can add customers before you create the charge by pressing the Customers button, which appears right above the 3 on the keypad when you're entering the credit card details. From there, you will find a form to input customer information and/or an email for a receipt. If you choose to enter any of these details, the customer will be saved in your Stripe account, and a receipt will automatically be emailed.
By saving customers, you can then reference these customers in the future to charge again, as their payment information will be saved securely at Stripe.
You can log in to your Dashboard to view your customers at any time: https://dashboard.stripe.com/customers