It's been around quite a while now but there doesn't seem to be any particular purpose to it yet other than to have a collection of friends listed in your profile. Maybe admin will add some special features at a later date.
We're still working on that section of the site. Eventually, your network of friends can connect you to people who are interested in the same health conditions ... and will serve as a compliment to the forums. For now, it's just a way to add friends and extend your personal profile.