Just add mention the bot in a channel and it will start to engage with you.
The feature team is still investigating this item and a decision has not been made yet.
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.