For a professional account, you only pay for team members (f.ex. colleagues from your company or institution).
If you share a board with a client or an external freelancer, they can access it as guests. This means they do not have to sign up for an account. They just have to state their name. Read more about the differences between "guests" and "visitors" here.
In case a visitor already has an account at Conceptboard, he or she is able to access your board with it.
Either way guests and visitors on your board are free of charge.
While you can share specific boards with anyone, projects (collections of boards) can only be shared with other members of your team.