Microsoft Teams Connect now allows external (federated) people to join group chats. Federated participants come from other Microsoft 365 tenants. Previously, federated chats were only supported for 1:1 conversations, but as part of the effort to prepare for the introduction of shared channels (also based on federation), multiple external participants can join a group chat.
https://office365itpros.com/2021/05/12/teams-group-chats-include-external-federated-participants/
