alosso:
If invited, the "Invites" button should blink (one on the left side of this window, one in the game client). Click on it, click on the invite, "accept", and you'll be taken to that game's lobby.
How to start a multiplayer game? It's in the FAQ =>
second but last video tutorial.
One more thing ... when I used Chrome as my browser, I had an issue accepting invites from the main web page. I would accept the invite, get the "bling" noise, but the game client would not load and the invite would disappear.
If you click the "Play Now" button and load the game client first, then whenever you get an invite, accept the invite from the game client instead of the website, you will be taken to the game's waiting room.