A setting in the Teams meeting policy controls whether users can access the meeting chat in meetings hosted by non-trusted external tenants. By default, the setting is On, meaning that users can participate in chat for any meeting they join in any tenant. If you have concerns about this aspect of meetings, turn the setting Off and define trusted tenants.
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.
Teams supports federated chat with other users in Office 365 tenants using a feature called external access. It’s similar to the way that Skype for Business federated chat works, except that you can’t use emojis. Generally things work very well, which is nice when you want to reach out and communicate with someone externally.