Hi @mark q thanks for posting in the community! While this is not displayed in the 'Manage Collaborators' section of the share dialog if you share a folder with a collaborator they should automatically have access to any documents and subfolders created within the parent folder. 

Any subfolder that is created should also have 'Shared Folder' in the collaborators list (see attached screenshot):

If you created a subfolder in a shared folder and the collaborators of the parent folder are not able to access this would you mind submitting a support ticket via this form with a URL to the folder so that we can take a closer look?

Feel free to also comment in this thread if you have any additional questions about this. Hope this helps!