Have the people who are unable to use the emailed invitation link been sent more than one email invitation? If so, its possible they are not clicking on the most recent email link. Once a new link is generated, the old one expires.
In the meantime, you can send them a personal email (or call them) with their username and password and the www.ftq360.net url. Or, have them download one of the Apps and add their login info there.