We go through a lot of eggs in our house which is perfect for the worms. The shells are collected and when we have about this much, they get pureed in the food processor into little granules, which are then added to the bin.
The egg shells need to be cooked til they are dry, which can be don in the oven or microwave. Once dry, puree away in a blender or food processor.
Egg shells are sprinkled on top of the worm bin. They work in two ways, one as grit to help the worms digest their food and two, by helping lower the PH level of the bin.
Be sure to check our your bin a day after adding the shells. Typically, you can find a lot of them having a party, rolling around in the shells. It's pretty comical.