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.”
When you start an online game, you can select “+” in the top right corner to add players from your friends list. For an online game with 4 players, tap on each player to add members of your friends list.
In the single player variant (of course) no other players can be added. But at least you can change the names of the computer opponents under "Settings" (cogwheel) -> "Game" > "Players settings" and so at least pretend that your friends are sitting at the table ?