In the Profile section, locate your client and open their profile. On the left you will see ' travel companions'. Select that and it will bring the screen to the section you need.
Click on to the red 'Add Travel Companion' button.
There you can add new profiles or type in existing clients and notate the relationship. This will show on their side as well.
^Note^ Syncing from Sabre will also create companions in the main profile.