Please note that this application is only a demo application showing how I would handle 2FA authentication.
And it is not about authorization. Anybody can create an user account or can send an "invitation" to anybody. This is something you probably not want in a real world application.
To login into this application you will need an Authenticator App. I've tested it with the following, but any App will do.
Source code for this APEX app is available at
github.
Or you can download the source from this application, just create an account and login.