Each Atticus license is individual, and it isn't recommended that it be used across multiple devices at the same time, or by multiple people, as that can cause errors in synching your content.
You can share the files by exporting the file to .docx, PDF, or ePub. Our programming team is still planning out the collaboration features that will allow co-writers, editors, beta readers, etc. to access your book and leave comments directly in Atticus, but these features are still a ways off.