This is currently in internal testing rings at Microsoft, and the feature team is targeting a release to the public in early Q2.
~Alex & the Teams team
We acknowledge the customer demand for Private Channels and have been hard at work on it. Private Channels is currently in preview with a select group of customers. We expect to release the feature to the public later this Fall, and will update here when we are starting to roll out to all customers. We appreciate all the feedback you have provided to us on this forum and thank you for your patience.
~AlexErik Schonsett commented
I would like the ability to have someone who is not a part of my larger team be able to join and participate in a single channel that is a part of my larger team. This is something that we used a lot in ASANA and are having a hard time with in Teams.