You can Invite a Friend by following these instructions;
Click on the invite button at the bottom of your screen:
A new menu will appear. It has three tabs: Rewards, Invite Player and Add Friends.
You need to select: Add Friends.
First you need to type the player name into the Search Box in the middle of the menu. This will search for the player and bring up any relevant player records.
However, this has not sent out an invite; it has just confirmed that you have searched for a player that exists on this world. You need to click on the player name and the following list of options will appear:
Click on Add as a Friend to send out a friend invite.