Microsoft Teams has just announced that you can grant access to users outside your tenant. You can read about it here:
Now being the eager beaver I am, I wanted this working asap. So I started invited people but for some reason they couldn’t gain access to my Team. They also seemed to get automatically removed from the Team after a period of time.
Turns out that external access for Teams is not enabled by default. To enable it you must go to your Office 365 Admin Center. Then select Settings from the left hand side.
From the menu that appears select Services & add-ins.
Locate Microsoft Teams from the list and select that.
In the Tenant-wide settings locate Settings by user/license type. Change the pull down to read Guest and the set the option to On as shown above.
The first time I looked, I didn’t change the pull down from the default of Business & Enterprise so I totally missed the Guest option D’Oh.
After I made that change I could indeed invite external users successfully into the Microsoft Team I had prepared for them!
So if you are having troubles like I was, check that you have enabled guest access as shown above.