I just stumbled upon this as a friend of mine and I are working on a business idea. While I already have an account with slack, I not just hesitated, but stopped moving communication to slack, because of this. I realize I should just go with it for the moment, but I really do not like to create more and more accounts. Just imagine a freelancer piling up accounts while working on different projects for different companies, while slack is widely adopted as internal communication in the corporate world. Just terrible.