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.
Please let me know if you have any questions.