Microsoft update the Product Terms site for May 1, 2021 by confirming that User SLs are not required for any user to join a Teams meeting or a Teams Live event. User SLs are also not required for external users to participate in Teams channels as a Guest with an Azure AD External Identity: https://bit.ly/3eGET3e.
If you’re new to Base and Additive CALs then you may find this new (October 2020) overview document from Microsoft useful. It explains software dependency licence requirements, Base and Additive CALs, external user access, and ends with some FAQs.
Find this document here: https://bit.ly/35fchcB.