An User is anyone who signs up and joins your company page. Typically authors are team members with a common work email. But you can invite people without a common email to be users as well. You have Unlimited Users.
An Curator / Admin is a User with rights to certain features. We have 3 admin types. Master Admin, Content Manager, and Content Users. Some of the things ADMINs can do that Users can not include: Request Stories from Guests, Approve Stories to be Public, Create Campaigns, Create and Publish Spotlight Pages, Access Premium Support. Each plan has a set number of admins. You can increase admins for an additional price.