I previously tried to sign up via a Google account but it required a 'company Google account' - I was slightly confused about the reasoning behind this.
It's designed for teams, Google auth is used to avoid having to manage invites etc. If you could sign in with any old Google account then it would introduce a class of problems with having to manage invites / user management / accidental duplicate teams.
Having said that, I do think it would be cool if signing in with your personal gmail account gave you a personal non-team wiki.
