There are two ways to find players and add them to your friends list, in the player menu and in the game history. Both can be accessed via the main menu. To do so, press the button with the dots in the upper left corner of the start screen.
1. If you are looking for a specific player, open the "Friends" menu item and select the "Add friend" tab. Enter the name of the player you want to add to your list. Now confirm with "Add friend".
2. If you are looking for a player you have already played against, open the menu item "Match History". Tap the name of the desired player in the game history to open his profile. Now select "Add friend".
When you start an online game, press the Invite button next to “Any Player". This will open your friends list. Now tap on the name of a player you want to invite. The player will receive a pop-up message, which they must confirm with OK. Now “Any Player” will be replaced with your friend's name. Go back to the player search and start the game.