When you go to the "Matching" tab (the one you land on automatically after login), on the right side you have a pane named "Friends". Everyone you have declared your friend (when it is a one-side relationship, this is often called "following" that player) and is logged in will be listed there, with their status (logged in, at a table, playing).
When they are on a table, you can join them there via a button. This is the way most people set up their games when they want to play with someone they know: One makes a table, the other joins it. That's also the way most people watch high profile games (from the Dominion League or other tournaments). You declare a good or interesting player as your friend (it doesn't have to be reciprocated), then join their table and watch the game(s) they play.
