You can add them by pressing "V" if you're on the pc version and then below you can type in his name and click on Add friend. Now you'll see him in a green name and not white around the world. You can also send them an invite to you link (It's like a group) by going in your friendlist now. If you go to this symbol You can press "Nearby players" this will show the nearby players but bare in mind that there are different channels where you can stay in to it is not guaranteed you two got placed in the same channel when logging in but do not worry, you can change between channels anytime!