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 three lines in the upper left corner of the start screen.
1. If you are looking for a specific player, open the menu item “Players” and select the search function, e.g. search tab or magnifying glass. 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.”
2. If you are looking for a player you have already been playing against, open the menu item “Match history.” Tap the name of the desired player in the game history to open their profile. Now select “Add friend.”
Another way to invite a friend to play is through the match history. You can find this via the menu in the upper left corner. Just tap on the name of the player and then on "Play". This only works if you have already played against this person.
When you start an online game, you can select “+” in the top right corner to add players from your friends list.