You can sign up for free, without having to enter payment details. If you want to add more than 10 members to your team, you have to upgrade to a paid plan first. You can do this yourself in team settings.
The Pro and Enterprise plans include a single flat fee for your team. Any applicable taxes will be added on top. To keep things simple, we will always bill you the based on the plan you have selected, even if you have less active users than the plan lets you have; to downgrade to a cheaper plan, simply update your team settings and your next charge will be lower.