You do need an invite. I see this as a feature - not a bug. It massively cuts down on spam. And also makes removing spammers much easier due to the fact that it

You do need an invite. I see this as a feature - not a bug. It massively cuts down on spam. And also makes removing spammers much easier due to the fact that it is not just invite only - it also has a public invite tree. So a spammer cannot just make 100s invites for sock puppets. They are trivially eliminated as well.