Starting in version 1.5.0+ you can automatically pass the users email to the Stripe gateway popup so they do not need to enter it a second time during checkout.
To enable this ability navigate to the plugin settings and change the status to Yes.
Go to Components > RS Membership > Payment Integrations and open the Stripe payment plugin. Change the Prefill Mail Field setting to Yes and save the change.
Now the email will automatically pass to the popup and it is not editable by the user (unless they return to the beginning of the checkout process and use a different address).