Check out this article to learn how to add a friend: How do I send a friend request?
Web
From the homescreen
One way to challenge a friend is to click on the Play a Friend button on your homescreen
This will take you to the Play page and show you a list of your friends and a search bar for you to type the name of the user you want to play against
Select the player and customize the game settings to your liking - If you and your friend are very far apart in rating, you may want to try an Odds Game.
Click Play to start the game:
Your friend must be online to accept a Live challenge; otherwise, the challenge will expire after a few minutes. For a Daily game, it doesn't matter if your opponent is online, offline, or in a game. The request will wait for them to accept it.
From a user's profile
Another way you can challenge a friend is by simply going to their profile, and clicking Challenge:
Or, click on their username anywhere you see it and click on the 
icon from the bottom menu
From URL
Another way to challenge your friend is by using a shortcut URL chess.com/play/username
Replace username in the URL with the member's username you want to challenge, then put the URL in your address bar. This will take you directly to a challenge for that member!
Mobile
Tap on Play on your homescreen to get to the new game menu and tap on Play a Friend
Choose which of your friends to challenge or if you know the username of someone you want to challenge, type it in the username field
Once you've picked your opponent, you will be taken back to the custom game screen, and you will now see your friend's name as your opponent
When you have all the settings as you like them, and are ready, tap Play and your friend will receive the notification of your challenge! When they accept, the game will start!
