When you need to discuss a private matter with several people, start a PM (private message) topic with one of them. You will see that you can add additional users to the PM topic as you are composing the initial message.
You cannot (yet?) add additional members to an existing PM topic, unless you are an administrator on that Discourse forum.
To start a PM, go to the user page of the user you wish to invite, and click the “Private Message” button that appears below their avatar. To add additional users, type the first few letters of the user name in the text box “Add a user”, then select the user from the autocomplete box, or type their full username.