A distinction must be made between Windows Users, and MailCOPA Users:
A MailCOPA User is a name used only within MailCOPA itself: it has no connection with Windows Users.
MailCOPA Users cannot see each other's mail, and for the most part have separate configurations under MailCOPA. MailCOPA users can log in from any MailCOPA client installation that has access to the MailCOPA server.
A Windows User is the term applied to the name under which you login to Windows. Some Windows Users can have administrative privileges, enabling them to make certain critical changes to the setup of the machine, including the ability to install (or uninstall) software. Other users may not have such privileges, and this will restrict their activities.
Under the above operating systems, you should install MailCOPA as an Administrator.
It will then be available to all users, who can (if they are a MailCOPA Superuser) create or delete MailCOPA users, and change MailCOPA users' configurations.
MailCOPA Users who are not Superusers are only able to access their own (or shared) emails, and can only edit their own user-configuration.