[tempaccount9473]

I can see your reasoning behing making people sign up, even if that will significantly retard your growth rate multiplier.

But there's NO reason to make people create a new account. Everyone hates making new accounts, and there are a zillion identity providers out there, from google to facebook to twitter to linkedin to github to yahoo (ad nauseum).

And even if it's too much work right now to configure your application to accept third party identities, you can still "sign-up" users by requesting an email address, letting them straight into the application, and then email them a URL with an embedded identity token that allows them to pick up where they left off.