Friends
Add a friend
Open Friends and use the Add friend tab. Search their username under Find players and send the request. They get a notification and can accept or decline.
Your list
Friends are grouped as In game, Online and Offline. Search your list with the box at the top. Press Name above the list to sort alphabetically instead.
Click a friend to open their profile. Right-click a friend, or use the dots on their row, to remove or block them.
Block someone
Blocking removes them from your friends. You can block from a friend's right-click menu or from their profile.