Starting September 11th, 2017, Microsoft Teams supports guest access for external users.
Guest access for Microsoft Teams uses a different guest license type that must be activated in the services & add-ons section of the Office 365 Admin portal.

Office 365 Admin portal notification:
When we make this change, the ‘Pick the license you want to configure’ setting in ‘Settings > Services and Add-ins > Microsoft Teams’, will now have an option for ‘Guest’ with a default value of “off” for the ‘Turn Microsoft Teams on or off for your entire organization’ setting.
When you’ve not activated guest licenses, external users receive an odd error when trying to access your shared team.
Link
Enjoy Office 365!
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