Google Voice and Google Fi

• Google Voice and Google Fi are not currently available through Chat:
  • Users who already have a Google Voice account can access Google Voice at voice.google.com, by selecting Voice from the App launcher, or by downloading the Google Voice app (Android, iOS).
  • Google Fi users must use the default SMS app on their mobile device to send and receive SMS messages.

Group Messages/Spaces 

• You cannot add external users to Group Messages (but you can use Spaces)

External Users

• You can add external users to Spaces but not to Google Messages: Create a new Space and check the box to allow people outside the organization to join.
• Users in Google Workspaces organizations that have not yet turned on Chat won't be reachable through Chat. Users can message any external users in organizations that have Chat turned on. 

See more information on limitations.