You can add egg shells as often as you eat eggs. Yes I assume that one of the benefits egg shells would be grit. I use a rolling pin to crush the egg shells. And sometimes I use a blender to get a finer powder. If you just crumble them in your hand it will take a long for the larger peices to break down and the worm would not be able to consume them. Crushed egg shells help to maintain a good P.H. balance in the bin and I've heard that the calcium may help the worms to produce egg capsules.

I collected around 7-8 eggshells, dried them in the oven and then used a rolling pin to crush them. I keep the eggshell powder in a plastic box and mix 1-2 teaspoons with kitchen scraps when I feed the worms.

we add egg shells regularly. It is especially good to give a good sprinkle of them to a freshly set up bin with bran new bedding. It gives them the grit to help break down their food.

Anyway, we let the shells dry out and then use an old coffee grinder to powder them. A sprinkle in the worm food can before taking it out to the worm bin is good. I will sometimes keep a container of the powdered egg shells by the bin to sprinkle over the surface when I add new bedding.

As many egg shells as you family can go though is probably quite safe and appropriate. If you have an outside sources for large amounts of egg shell, that could be too much and probably better spread around the garden or compost bin.

Here is a link that might be useful: My Bins

ALL of them. I try to crush them whenever I can, but they sometimes get thrown in whole (err, halved) by my wife. That's not a big deal. I find them when I do worm/poo separation and crush them up then by hand. The shell do a great job of moderating too much acid in the bin (vinagar smells) and add tons of calcium to the compost that my tomato plants just love.

Mike

I let the shells dry first, then use a mortar & pestle, which works quite well, to grind them up fine. Then I keep the shell powder in a little zip-loc so it's handy and all ready to add.
jt

I've put eggshells in the microwave for 30 seconds dries them out and makes them brittle then I crush them by hand and chuck them in the bin. But I don't add many. maybe one a year, if at all. Just personal preference. The worms don't seem to mind at all.
I assume they get all the grit they need when I add the dust I sweep up off the floor.