I drop an egg in the bowls about once a month. Jade eats some of the shell, Dozer (10 year old RottenChow) no shell, Duncan (11 year old RottenChow) a little shell. Even thought the shell wont digest the calcium can still be absorbed by the dogs digestive system. Be careful on a growing pup with kibble or other calcium supplements.
When I make my dog food I scramble 6 to 8 eggs in with 4 lbs of hamburger and about 3 cups of rice, I grind the shells up in a coffee grinder until fine and add that back into the mix.