To get an io9 account, you first have to make one comment.
Write your comment in the box at the bottom of the post you want to comment on, then click "share."
Tip: Make your first comment a good one, because our army of comment moderators will decide based on that comment whether to let you have an account. Don't make your first comment something like "yeah!" or "u r dum."
After you've submitted your comment, a dialog box will pop up like this one:
Click the "sign up" button and fill in your name and contact information. It may take up to 24 hours for your account to be approved.
Tip: Before you fill in your name, don't forget that your comments are public, and can be found with search engines. So be smart. If you don't want your boss or your family to find your comments about Star Trek pinups, use a pseudonym or nickname.