You'll simple have select Add Members in the menu next to the team name and add the guest's email address. They'll then receive instructions on how to create a Microsoft account and to join your room. The feature, which was originally announced in September 2017, will begin rolling out next week and will be available in every Team's room within a couple of weeks.

The ability to add anybody to a chat room will already make the app more competitive, but according to Petri, Microsoft is cooking up another feature that could turn Teams into a veritable Slack rival. Petri says Team's latest developer preview contains several references to a freemium tier. You still need access to at least one paid Office 365 account to be able to unlock all its features, but you can at least give the app a try if you're still deciding which team messaging app to use.