Follow these steps to add players to your friends list: 

1. Press the menu button in the upper left corner of the main screen 

2. Open the menu item "Friends"

3. Select the tab "Add friend"

4. Enter the name of the player you want to add to your list

5. Tap on "Add friend"

When you start an online game, you can tap "Invite" to add players from your friends list. Alternatively, you can tap on the name of the player in your friends list via the "Friends" menu item and the "Friends" tab and challenge them via "Play".