1. Click in the Friends feature in the bottom of the screen;
2. Click the icon at the right with the "+";
Be aware that if your account is connected to Facebook, your Facebook friends that have their account connected to Facebook will be automatically shown.
You can also invite your friends through Facebook or WhatsApp
3. Type the name or the Unique ID of the player you want to add and tap on "Search";
4. Tap on "Add Friend"