ZPM users can either be users created through WordPress or through the frontend manager option, which allows you to create users quickly and efficiently (and is only available to administrators). You can find a guide on how to create users for WordPress here.
Once a user is created, you can set their user role in the WordPress dashboard under the Role option.
ZPM creates 4 new custom roles which are ZPM Administrator, ZPM Manager, ZPM User and ZPM Frontend User which are all customizable by administrators to edit their permissions.