If you have never used a professional portrait photographer, here are some factors you should be aware of and consider:
The Photographer: Make sure your photographer knows how to shoot portraits. Beware of photographers with lots of expensive equipment, but no real experience in photographing portrait work. Check out their advertising and website and see if they say anything about portrait photography; or are they really a sports or product. Your photographer should understand posing as well as proper shadow lighting arrangement. Also, for a larger group portrait, be sure you have found an experienced group photographer. A experienced portrait photographer will have the proper lighting, light modifiers, as well as the knowledge in tricks of the trade to properly accentuate wanted perfections while hiding your unwanted imperfections. Most of all, be comfortable with your photographer.