Project.co is based on the premise that 2 companies come together to work on a project; your company and your clients company.
Each company can have many different people associated with it. The people associated with your company are called internal users and the people associated with your clients company are called external users.
Internal users are people within your company. This would include you, your team and any contractors or freelancers you would like to contribute to projects and manage projects.
External users are any users that work for your client. These users can contribute to projects and see tasks but they don’t need to manage projects and as such they have a reduced permission set.
See more about user roles and permissions here.
The Project.co pricing plans are based on the number of internal users you have within your account but all allow you to have unlimited external users and companies.