Welcome to the WGT, Richter4MVP!
You can either bring up someone's profile by clicking on their name and then click the INVITE FRIEND box or you can search for people through the WGT PLAYERS section and do it that way.
Another way to do it is to get in game with some people and click on their name at the top left. It'll bring up their in-game profile and there will be an ADD TO FRIENDS button there too.