Presently, to create a new Team, you need to have at least one member who can be designated an administrator. This person needs to already be a Bitbucket user.

The behavior should be such that if someone wants to create a team, they can designate a user's email address as the team admin and the admin can accept that. As the user creating the team will know the account credentials to log in as the team, there is no reason why there should be a requirement that the admin already exist.