External access is not available in Microsoft Teams preview. We are
working on several ways to expand the value of Microsoft Teams to more
diverse types of organizations and teams, including the ability for
guests outside the team or company to participate in a meaningful way.
Its a huge demand from our customers, that they can invite partners / external people to their Teams. Its a showstopper as it is now, because customers want/demand this, to implement it to their organisations.
+1 on the importance of this request. Most of the scenarios I want to use teams for involve being able to work with people who don't currently have O365. Allowing them to participate in Teams would be a great way to introduce them to O365.