I just signed up myself, and the sign-up notification confirms that they are sending invites to readers in waves -- so as not to brick the whole system again -- so don't worry if you don't get your invite exactly on April 11. That said, you only have 48 hours from the time you receive said invite, so don't procrastinate.
Go sign up! Get free comics!
Joey is a Senior Editor at IGN and a comic book creator. Follow Joey on Twitter @JoeyEsposito, or find him on IGN at Joey-IGN. If he could, he'd run away to live amongst wild cats for the rest of his days.