Starting September 11th 2017 Microsoft Teams supports guest access for external users.
Guest access for Microsoft Teams uses a separate 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 user receive an odd error when trying to access your shared team.
Enjoy Office 365!