If you wish to play against friends and want to add them to your friends list, you need to know their player name and hashtag number (Ex Maria#1234). Simply type their player name plus hashtag in the search bar at the top of the screen when you're in the menu that says "FRIENDS" (- the fourth button from left at the bottom of your screen). Once you're friend has accepted your request, you'll be able to see when your friend is online and available for matches.