You can join an existing account online here. Under Join as an individual select "I've been invited to join an existing account."
Enter the email address of the account you would like to join. Once found, you will be taken through the application process.
When joining an existing account, the account holder will be responsible for all billing matters.
Once your application has been submitted, the account holder will be able to accept your request to join their account. Once accepted, your application will be processed by Zipcar.