You can add seats to your account by clicking on the Members page and clicking the Add member button.
All team plans include 3 users per account. The admin is the only user who can invite new members. After typing the email address press enter and you will now see the email address has been recognised. Press Invite and the new member will receive an invitation.
Adding seats to your plan takes effect immediately.
Adding more than 3 users to your team plan costs $75 per additional user per month. To adjust, click Change Seats and increase the number of seats to your desired amount.