Private channels are useful for closed conversations between project team members. They can only be joined by invitation and are only visible to their members, as well, they cannot be searched in the main directory. Administrators, channel owners, channel moderators and channel members can invite others to join the private channel. 

Click on the Pen icon (located in the top right of the left sidebar) to create a new private group.

Adjust the settings for new channel. 

To make the channel private, switch on the setting for Private Channel (the button will turn green).

Add a name for your channel and invite users. Note: new users can always be added later.

Click Create to save your group settings.