Your member id is just your email address. You will have a different one for the sandbox and for production. If you search for sandbox on the paypal site you will see where to sign up for the sandbox. You need to use a real email for the sign up but part of the process you will create a fictitious business user and a fictitious personal user. Both will be email addresses generated by paypal sandbox. You can view the email sent to these fictitious characters on the sandbox site. They won't be real.