I was able to successfully add users who immediately made an account. I am trying to add another user who has had an account on wordpress for about a year now but it won't let me add him. It says "That email address does not belong to any user here. Would you like to invite that person to use WordPress.com?"
But he can still sign in with the e-mail address I used to add him initially.
If I choose to invite him it prompts him to make a new account.