moberlefs on "invite as contributor w/out forcing them to set up their own blog"

I'd like to invite colleagues to join a new site as contributors. Luckily, I tested this process by sending an invite to myself (to a little-used email address of mine).
I followed http://en.support.wordpress.com/adding-users/ and "To add someone who does not have a WordPress.com account:". I got the email + link to accept the invitation. The link took me to a WordPress page to sign up with username, pw, and a choice to create my own blog or "just a username please". I checked the latter and then "Next" and it still tried to create the blog username.wordpress.com (it couldn't, because the username.wordpress.com URL already existed). Will all who are invited or sign up as a subscriber be forced to also create their own WP blog? I just want the sign-up process to be simple. If it's not, this isn't going to work at all.