How to connect a project to an existing folder/project?

Essentially, I need to share a sub-folder of a project as it's own project with collaborators outside my own organization. These collaborators do not have permission to see the larger project, only the sub-project. They also need to be able to make changes to, and have access to the latest versions of, the sub-project. I can't seem to find a way to do this.

I have currently shared it as a "Partner Space" so they can at least see the files but this seems to not auto-update and doesn't let them contribute from what I have read, making it inadequate for actual collaboration.

I also have the large project on my Dropbox (with the rest of my work that isn't CAD related). It was working fine until I now need to work from home, and GrabCAD won't let me associate the project on my home computer with the folder in Dropbox.

I have attempted "Manually Connect Project" but it won't let me because "A previous mapping already exists at that location". Creating duplicate local files seems like a waste of disk space and unnecessarily complicating computer organization.

So essentially, the question is: How do you connect a project to an existing folder that may be the directory (or sub-directory) of a project?

It seemed like a fundamental function of a project sharing solution like Workbench, but I haven't been able to find a way.

Thank you in advance.