One may have received a link from a participant who forgot to mention that the site is still in its private beta. And a login suggest "accessible". Having the "this site is currently in private beta, you're to late to sign up" shown after creating an account is a bit annoying.
Impossible -- we don't know who you are until you attempt to log in.
We need to know if you're on the whitelist or not, and that requires you to authenticate (show some form of identification).
edit: I see your point now, and I've added
Warning: this site is currently in private beta for (n) more days. Unless you committed to its Area 51 site proposal, you won't be able to log in!
To the login page during the private beta period.