1⃣ In the left menu of the forum, click on "Private Chat" :

2⃣ Then click on "Discuss privately" :

3⃣ The usual discussion editor opens. Before composing your message, please select your recipients by clicking on "1 Recipient" :

4⃣ Select the users/groups you wish to add in turn by typing their name in the search field, once you have selected all desired recipients then confirm by clicking on "OK" :

5⃣ Validate your private discussion by clicking on "Post Private Discussion" :

Note : As with normal chats, you must enter a title for your private chat.