What you can do is upload the files to Box or Google Docs via Clarizen. Then, you can invite the users through Box or Google Docs account. It is the same step as adding related files in Clarizen. However, we do have a new license that is coming out early next year that will allow users that are email only to view files. It is called the Collaboration License which will be available soon. Please let me know if you need some help on how to upload files to Google Docs or Box via Clarizen.