If you go to the game's profile (and you're a GM), there's a yellow button that looks like 2 people with a plus over their head. You can click that and it'll bring up a thing where you type in their username, and send the invite from there.
Click on your game link, then Game Profile tab, its the first on the left.
Inside here, there will be 3 smaller buttons in the center of the page, with "Archive" button just beside them. The third button, the one just to the left of "Archive" is the one the DM clicks to add players to his/her game. This sends your players a PM for them to accept or decline.