The folder that the desktop release of the Twine 2.x application uses to store it's project HTML files differs depending on a number of things like the Operating System being used and the Laungage that the OS or the application has been set to.
You can determine the location of this folder by selecting the Twine > Show Library menu items, which should open your OS's File Explorer with that folder selected.
You have a number of options available to you for sharing the contents of the above folder:
1. You could move that folder to a portable media device and then create a symlink replacement in the original location (with the same name as the moved folder) that references the new location of the folder.
eg. (for Win10) If the folder was originally located at C:\Users\XXXXX\Documents\Twine\Stories and you moved the Stories folder to somewhere like F:\Twine\Stories then you would create a symlink named Stories in the original parent C:\Users\XXXXX\Documents\Twine\ folder like so.
mklink "Stories" "F:\Twine\Stories"
2. You could install a Version Control System like Git or Mercurial and then create an repository on one of the online sites like GitHub or BitBucket, you could then use the VCS software on the (above) Stories folder to track the changes you make to your project HTML files.
warning: You may need to close down the Twine 2.x application before you commit and push your changes up to the online repository.
3. You could create an account on a File Hosting site (like Dropbox) and then install it's associated Desktop Application (like the Dropbox App) to help move the monitored files between your local machine and the file hoster.
warning: (on Windows) Because the default Stories folder is part of the User system and therefor a virtual folder, I would suggest first using the technique mentioned in point 1 to relocate the Stories folder to somewhere else on your local harddrive, and then use the File Hosting Desktop Application to monitor the new location instead.