go to the WGT community tab at top of page, and click on WGT Players. Enter his WGT name in, and hit return, his name should appear in green.
Click on his name, you should go to his profile page, on that page above his name and avatar you will see a number of options, invite to play, add as friend, click on add as a friend and a request will be sent to your mate, he accepts, job done.
cheers