if your friend is online they will appear in the "friends" tab...just click their name and click "challenge to a game"...
How do I challenge a friend to a live game?

It explains in detail how to challenge your friend to an online (moves per day) game aswell as a live chess game.

Here's the way I do it: arrange with your friend via messages to play a live game together. Then both of you choose Live Chess from the Chess.com menu bar, and click Play Live Chess on the My Live Chess page. Then click the Friends tab on the gameboard page, and you'll see your friend's name, and an option to challenge them to a live game.
There must be a way, I just can't seem to find it.