After a long time in development, Rockstar Games have finally launched the friends and message wall features in the Social Club. To get started you'll need to build a friends list, you can do this by automatically adding them from your linked Gamertag/PSN profile, or if you know their username you can just type it in and add them. Having a friends list then opens up the message wall:

Once you have created a Friends List you can begin using the Message Wall. The Message Wall is a unique verbal and visual means of mouthing off to your Friends – including a direct message system and customizable personal graffiti wall. Communicate, talk trash with your Friends, and decorate and/or desecrate each other’s Walls at will. Look for more additions to the Friends System and Message Wall in the future.