Collaboration PLUS is based on user groups. Each group has certain access rights. Depending upon what group you are assigned to, you may be able to add or delete employees. The base group is "Registered." This group is typically an employee. A Manager, however, has the rights of a "Registered" user plus the ability to add documents. The "Administrator" has even more access rights; to add/delete employees.
This screen shot shows a typical user group configuration:
Note: The "Public" ad "Guest" groups have no bearing on access to your Collaboration PLUS portal. When you first access the site, you are in the "Public" state and can only login in - nothing else is available. Once you login you are granted access based on your user group.