What happens with users who left our company, can they use the email address they had? Can they then find a way to log back into our flows and their old conversations?
Users can only be part of your organization if invited by an organization owner.
If users leave your organization, they may create personal trial accounts, they can use any available email address. Assuming their old business email address is available then they can use it, but that will not link them back into your organization. They will not get access back to your flows, not restore their old conversations and certainly not charge a seat to your organization.
Users can only be part of your organization if invited by your organization.