You can use two factor authentication when the Google Authenticator module has been installed.
First install any OTP app on your mobile phone. You can use for example:
After installing an OTP app on your phone go to “My Account” -> “Account” and click on the “Enable Google Authenticator” button.
You will be prompted for your Group-Office password. Next you will be asked to scan the QR code with your app to register Group-Office. Enter the code from the phone to confirm you’ve set it up and you’re done.
While this module is called Google authenticator it doesn’t need a Google account. It does not interact with Google when logging in in any way.