If you have enabled Find My Mac, the guest account is automatically created. It is vitally important that you let this account remain. It is no ordinary Guest Account. It only permits the user of this account to access Safari, so that in the event a thief steals your computer, it is hoped that he will use this account to reveal your stolen Mac's location.
You can remove the account by disabling Find My Mac, but as I said, I don't think you would want to do this.
If you have not enabled Find My Mac, then the ordinary guest user account is managed from within the Users & Groups pane of System Preferences.