They might be able to do it for a day or two, but eventually something gets in the way. Our job when writing that first draft is to get down the story or the message or the teaching—depending on your genre. Review what you wrote yesterday to refresh your memory.
These are practically free, so if you insist on only allowing people who have paid to read your writing, you are in for an even more uphill battle than writing a book normally is. The legendary Hollywood screenwriter Dalton Trumbo often wrote and edited in the bath as it was the only way he could get away from his children.
Always remember: Done is better than perfect. Now that you know how to write a book, what are you waiting for? However, just like any other seemingly insurmountable task that others have accomplished, the journey from start to finish is best handled one step at a time.