If they are pullets still then soft shelled eggs is normal. Some birds take a month or so of laying before their reproductive system is fully developed. Strange things can occur like no shells, no yolks and even an egg inside another egg or two eggs attached to one another.
It could be that one girl is only laying every three days and all her first eggs have soft shells. Layer feed has enough calcium and likely the problem will resolve itself in another week. But adding egg shells back to the flock is fine and easy. We'd just microwave the shells after use for a minute on high to sterilize them then store in a container until a decent supply then lay them on a cookie tin and using a dough roller turn them into pieces.
