If you have time this weekend, do a lot of spying on them. Remember who is in the nest when, check for eggs often. You can narrow it down considerably just by observation. If your chickens aren't all the same breed, learn the differences in the appearance of each girl's eggs. They may vary slightly in color, shape, size. For the soft shells, make sure they get plenty of calcium. You can feed them crushed egg shells and oyster shells from the feed store.
You have to keep going in the coop and watch. If you have hens that look the same and you want to mark them put dark nail polish on a toe ( not red). I was able to know who's egg was who's by the shape and color by watching who was sitting in what nest and finding the eggs.
If these are just pullets starting to lay, the soft shells might be just their systems getting into the groove. If it continues then I would give oyster shell as a free choice item. Most feeds have plenty of calcium, so you don't want to overdo it.
If you are really wanting to know without having to watch them constantly to find out who is laying which egg, take some food coloring and put a drop on their vents. Use a different color for each girl and then when they lay, there will be a streak on the shell of that color and you will know who's laying what.