Simple and short guide to setting your lobby to private, for those who want to play solo or with their friends only.
How to set your lobby to private
How to do it
While looking for a mission or the world map, click on your avatar , bottom right:
Move your mouse/cursor towards the top right to highlight the area:
Click or press A/D for Public or Private, depending on what you need.
As you can see this is also a window to invite friends.