First it should be noted that only the admin can add stories to certain folders, such as the Canon stories as well as Robipony's favorites in the Fan section.

So there is a bug here on FiMFiction that has yet to be fixed where if a story is empty you can't add the story to the folder from the group folder and must instead add it manually.

Here is a guide on how to do this.

1) Find the story you are looking to add.
2) At the top of the story next to the Like/Dislike Bar and Views meter is a button with a down arrow. Click on that button.
3) A list of options will appear. Click the option: "Add to Group"
4) Find the group you wish to add the story to (either by scrolling through your collection of groups or typing the name in the search bar), in this case look for: Monster Mares and You...
5) Once you have click on the group a list of folders will appear. Add the story to the folder that best fits your story.