Q: HOW TO COLLABORATE SECURELY USING IPHONE OR ANDROID

The easy answer is to use Google Drive with Google Docs (doc, sheet, etc.) Google maintains encryption in transit and at rest, and multiple users can work on the same document at the same time.

Original Post: https://www.quora.com/Is-there-a-simple-program-or-website-that-will-allow-me-to-collaborate-encrypted-documents-with-others-that-are-using-iPhone-or-Android/answer/Marc-Mintz-4