I Suggest an Event like Stories mentioned
Here's what I did:
Create an Event and use the current day/time or like 5 minutes from now.
Example format: December 11 2017 12:00pm
For your title and description just put something random (because it can still be viewed by anyone at this point) I put "test"
Make sure you set it to anyone can join and check the require approval box ( private means only for you)
Once that is set, wait for the time to expire.
The reason for this: if you go to view all events and the event has not passed, it will be publicly available for everyone to see
http://forums.d2jsp.org/events.php?a=2Make sure you no longer see your event in that list, then you can start linking the people you want to your event and start your private conversation there.
I'm not sure about timeframe but i think within an hour after it expires it will be removed from the list
At that point you can edit your title/description properly now that no one else can see it.
The only downside I've found in this method is that all users can edit/delete their posts at anytime and it won't show any history of it. So if discussions get heated and people try to respond/quote each other it could get messy lol