I feel like there's some room for an invite-only period that's useful, but it needs to be short. Unless you've somehow got an amazingly complete test suite, you're likely to run into scaling problems and show stopper bugs with early users, and limiting user count temporarily allows you to see and fix some of those before you become known for your failure conditions. But you also don't want to linger in invite-only, and lose the hype. IMHO, maybe 2-6 weeks of invite only is fine.