Invoice Posting Authorizers - Can't Add User

I am trying to add a user to the Invoice Posting Authorizers screen, but the user does not come up when I start to type their name. Supposedly the only two prerequisites for this are that the user exists in the Person window - which they do. And there should also be authorization group IDs defined in the Authorization Combination Rules window - which there are. And this user is parts of that authorization group. Why doesn’t their name appear when I try to add them?