They have to be logged in to the live chess server, then you just click on their name and challenge them; oh, you have to select the "friends" tab first. It's covered in "offering a challenge" in the link:
https://support.chess.com/customer/portal/articles/1444847-how-can-i-play-a-game-in-the-live-chess-
How does one play LIVE CHESS with a friend?