How to add a friend
Click on the friends' icon on the bottom left corner.
On the next screen, you can see the friends you currently have (the account in the image has none) and if you press the "invite friends" button you can send invites to friends. You just need to search for their Unique ID.
After you've sent the request, you'll see the player on your friends' list.
How to remove
When accessing your friends' list, you'll see a little cross on each player's profile.
You will need to confirm that you wish to remove the friend and restart the browser/app in order for the friend to be removed.