To access the social menu, click on its icon 
on the right side of your screen while in GUI mode.
The first window in this menu is the friends list. Here you can search for players and then send them a friend request.
You can also browse the list of your existing friends.
To send a friend request through this window you first need to toggle the search function by clicking on the toggle at the bottom.
Then you must type in the name of the player you want to send the friend request to. Typing a minimum of three letters will initiate the search and return any valid results.
To send the request, click the plus sign next to the players name and then click the Send Friend Request button at the bottom.
Through the friends list you can also summon any friends to your location.
Summon friend is a special form of summoning that any player can perform to teleport any player from his friends list to his location.
To initiate summon friend the player needs to have a portal shard in his backpack. Once the summoning is initiated, the player will need to stand still for 2 minutes while the process is in progress.
At the end of the summoning timer, the player that is being summoned will receive a prompt which will need to be accepted for the teleport to be completed.
Once accepted, the summoned player will need to stand still for 20 seconds to finalize the process.
Summon Friend 
Portal Shard
The second window, titled Friend Requests, will list all incoming and outgoing friend requests you have pending.
The blocked list will list all the players you have ever blocked. You can remove players from the blocked list by clicking on the icon next to their name.