Admin managers and managers of your environment
Paid licences include admin managers, managers and collaborators in your environment. You can manage all active licences from your settings in the "Accounts and licences" tab.
It is useful to add admin, manager or collaborator managers to your environment when you want your collaborators to be able to create projects, validate tasks, modify a report,.. on their own.
All external contributors (customers, contractors, etc.) are completely free.
TIPS : If you want to add collaborators to your projects free of charge, you can add them as contributors: they will have read access to the projects, documents, notifications,... linked to them only.