To invite new users you must have an admin role, the other user roles do not have the faculties to add or remove users. If you are an account administrator, you must click on your name and go to the option "add more users to your account", then enter the email address of the person you want to invite and click on "send invitation".
Why do we offer unlimited business users? Because we have found that selection processes are more efficient when more people on the team are directly involved in it. For example, it is much better when the area- or person- asking to hire a profile can observe and participate directly in the selection process, instead of waiting for the recruitment area to deliver reports on the applicants.
Learn more about the advantages here.