This has been a long awaited capability: creating Teams channels you can share with external users (guests).
Well, the wait is over. You can now create shared channels to collaborate with external users without sharing your Teams.
NOTE this option is not (yet?) available in Teams on the web, Teams mobile or even the Teams administration portal; it is only available on Teams client
This capability is currently in public preview (you can join the preview by following these steps https://docs.microsoft.com/en-us/MicrosoftTeams/public-preview-doc-updates/)
To create a shared channel, use your Teams client on Windows and access the Teams on which you want to create such shared channel
When creating the channel, choose the Shared option
When creating the shared channel, you will have an option (checked by default) to share it also with existing Teams member
Once created, the shared channel can be identified with the “S” icon on the right side of the channel name
Administrators (Azure AD) can keep control on how the shared channel can be used by managing conditional access (same as with SharePoint Online). B2B settings (External collaboration settings) and/or External identities\Cross-tenant access
Enjoy this is new collaboration capability in Teams