Traveling and on a bad connection right now, so apologies if you get two copies of this post.
Slack. On WhatsApp you can't have threads, and sharing short code snippets is a nightmare. Also, your phone number becomes accessible to all in the group. Telegram is better than WhatsApp, but for large study groups Slack is superior.
git. I don't think we'll ever need to share large datasets which aren't already publicly available. PDF Papers or course notes in Markdown can be easily accessed on GitHub.
15 pages/week. I'm sure many of us are already following other courses and/or working full-time.