This is horrible--steals/hacks your email
This is horrible. Like ABSOLOUTELY HORRIBLE. They hide in your friends email, saying to boost their rep. But they only allow sign in with Google. Then, somewhere in their consent/terms, they make it so that they can send emails in your name. Then, all your contacts (regardless of who they are) get an email telling you to refer them. Then when your friend signs up, the process repeats all over again.