You'll need to select the 'private' toggle option when creating the session
Once you have created the invite only session, scroll down and find the Private/Public toggle option within the session settings menu.
When a session is private it is only viewable to those attendees that are 'invited'. You can control who is able to view the session by scrolling down to the attendees tab and adding in select attendees here.
How do attendees access a private session?
For attendees who have been added to a private session, there are two ways that they can access and view the session.
Playlist
The first option is to add the session to a playlist which is visible on the homepage. That way, when the attendee is added to the private session it will appear on their event homepage. The session will only be viewable to those attendees that have been invited.
My Agenda or Homepage now options:
If you'd prefer not to have the session appear within a playlist, you can enable the my agenda option (found within the general settings section). Once added, the session will appear in the selected attendee's agenda, which when clicked will give them access to the private session.