You can send a friend request a couple different ways. If you're playing at a table and want to add someone, click on their seat and then click "Add as a Friend" in the popup window.
Or, if you go straight to their profile page, you'll see an "Add as a Friend" button on the right side of the page under their stats.
When you receive a friend request, you will get a notification in the top menu bar. Clicking the icon reveals a list with your requests and you can accept or reject the friendship.
You can also manage your requests from this page.