Our current system is not set up to have multiple authors work on a single FlexBook, though you can still do so.
We generally recommend that each person work in their own account, then email a link to their work to a single person who can add those sections to a main FlexBook. It is important to note that at no time should multiple authors edit the same book at the same time using a single account. This will result in versioning issues and possible loss of work.