To add or remove team members, click the Team tab on the left:

You can add members using the "Add Team Member" button. Actions allow you to log into the account directly or remove an account from the team:

Once a new user has been added, they'll automatically receive a welcome email to verify their account and set up a password. Please be sure to check the junk/spam folder as it is very common to have email delivery issues due to automated filters.
To add additional seats to your team, please reach out to Support.