In the Inquiries section, click on "Invite colleagues" in the bottom left corner:
This will take you to the Admin settings, where you can add one or more users.
If you run out of seats during your trial period, you can add more seats to your workspace.
These will only incur charges after your trial period has expired and payment information has been added. It's therefore free to add users during your trial, but these users will be added to your subscription, so your first charge after the trial expiry date will include those add-ons.
If no payment info has been added, the trial will expire automatically after 21 days.
Users with Administrator rights can convert the trial accounts into paid accounts by following these instructions.