Ace of Arenas offers several ways to add friends. One way is to play a match with someone. On the results screen that appears at the end of the battle, there will be a small icon next to that player's name. Simply tap this icon.
The person you just played with will be added to your friends list.
Another way is to exchange Player IDs. To do this, first go to your Player ID screen. Then tap the "Add" button at the bottom left.
Enter the Player ID of the player you want to add as your friend. That player will be added to your friends list.
You can also scan another player's QR code to add them as a friend. To do this, go to your Player ID screen and tap the "Scan" button at the bottom right.
Align the QR code inside the frame, and it will automatically scan your friend's Player ID and add that player to your friends list.
If you happen to have an image of your friend's QR code saved on your device, you can also tap the "Scan local" option to scan it from your own photo gallery.