This article contains steps on how to join a teams group via Teams once invited as an external guest.
Receiving Invite
Once you've been invited you should receive an email stating that "You have been added as a guest to <Organization Name>". The email will have a button that takes you to the Microsoft Teams website (as shown below).
On the Teams website it will give you the option to launch the Teams app, download the app, or to use the web app instead.
Selecting Organization ("Org")
If you are logging on to Teams for the first time on the device, it will give you a prompt to select your teams organization referred to as org.
From the options given you would select the one with the external organization name followed by "Guest".
Switching Org After Signing In
If the prompt does not come up and instead you see your usual chats, switch the organization you are viewing by clicking on your profile picture on the top-right of Teams and selecting the correct Org.
Once switched, you should now be able to see the teams shared with you and access any content within them.