They just need to get used to you. Try sitting very still in front of the tank for around 10 minutes a coule times a day. After a week or so of that, start moving very slowly while you sit in front of the tank, it will take some patience on your part, but eventually they'll get used you being there and won't hide as much. It also helps when they start associating you with food, mbuna will do anything for food.
And like Nick said, don't be surprised once they are coming out for you, they'll hide from anyone else. Mine always hide when I have company.