In this menu, you can view the friends you've added, the pending friend requests and how you can manually add other players. The maximum number of friends you can keep in your list is determined by your rank. The higher your rank, more friends you can add. Using the characters of your friends in your list gains twice the amount of Friend Points which you can use to perform Friend Point Summon and get new characters (Training fodders)
You can also receive friend request from other players. You can only have 30 pending friend requests at a time. If you maxed this out, no one can further send you friend requests until you reject or accept the pending ones.
You can also manually send friend requests to your friends, as long as you know their ID. If they need to do it the other way around, you can find your ID and give it to them as well.