To add players to the friends list, press the "Player Profile" button on the main screen of the app.
If you are looking for a specific player, open the "Players" menu item and select the search function. Enter the name of the player you want to add to your list. Tap the name of the desired player in the results list to open their profile. Now select "Add friend".
When you start an online game, tap "Create room" and then "Add friend".