You can have a quick picture taken as your group enters any of the parks.
For something a bit more formal you can contact Disney Event Productions Photography. They do family portraits for large and small groups.
Our family used them in 2000 for a group of 6 and we were very pleased with the final product. They let you choose your location - somewhere on WDW property, then a photographer meets your group there and take the pictures. Our photographer spent about an hour with us. Pricing was similar to family protraits we had done at home...sitting fee, etc.
Disney Event Productions Photography: 407-827-5099