In the Files section for your group, you can now create files that are links to files stored in Google Drive/Docs, Dropbox, Box or Microsoft OneDrive. Look at the new + New dropdown in the Files section.
This option lets you input a URL for a file. For example, in Google Docs, you can get a public or restricted URL to a file by clicking the Share button.
Selecting the different services just changes the icon used to display the file. It has no other effect.
