Two items:
The big "Invite your friends" button below the "Play now" button is meant to write an email to a friend who is not yet member of WGT. Once he accepts he will become a WGT user & both of you will receive a gift by WGT and you will be WGT friends in the friends list.
To invite WGT members to join your friends list there are two options:
- go to their profile page and use the smaller "+ add as a friend" button (right hand side),
- in a multiplayer game, click the little "+" sign in front of his name.
Both options will send a "friend" invitation to the player which he can accept or decline.
Once players are in your friends list you will be aware of their online status, blue or green for "free" => you may invite them for a game, red for "busy", grey for "offline".