Here are two ways you might be invited to join an existing Microsoft Teams free org.
Select the Join Teams button on link in the email.
Review and Accept permissions.
You'll be prompted to choose between opening the Teams app or the web app. Pick one and Teams will open with you signed-in.
Via a link
Select the link or paste it into a browser.
Fill in the form with your name and email, and select Send request.
The admin will see your request and needs to approve that you can join the Team. Then you'll receive an email with instructions on how to join the Teams free org.