WARNING! Car2go security is garbage and they will make you pay when it fails.
The card they issue you can be used to access any car2go. Even though a PIN is required to start it, if any vandalism or theft happens in the car, they will take no responsibility and charge you for it.
ALSO, they don't do any real monitoring of fraudulent behaviour with the cards. My stolen card was used 4 times in 10 minutes to access 4 different cars without entering the PIN. They didn't notice.
They contacted me three days later when another user reported a missing key. Hopeless! If the credit card companies were as lax with fraud, no one would use them. I recommend you don't use Car2go until they fix their security model. I certainly won't be using them anymore.