As you may expect, the Friends window (default "N") is fairly standard to most MMO's. The only real key difference comes in the form of a messenger type whisper window that is accessible only to friends.
This is fairly straight forward process, but in the case of Black Desert you need either their "Email" or "Family Name" to add a player to your friends list. Simply click the "Add Friend" button and enter the proper information.
As soon as you, or your friend accepts the request you will have access to any additional features provided (such as a private whisper chat window) if so desired.
This process is almost identical to adding a friend, but instead you look for the tab labeled "Requests" and click on any outstanding requests, then click on accept.
For the socialites out there, a feature is also provided that allows you to group friends based off of a title of your choosing. This allows you to easily organized large friends lists into categories for ease of use.