Guest support is the ability to add someone outside one's own organization to a team.
Starting the week of March 5, Microsoft will begin rolling out the ability to add anyone with a business or consumer email address as a guest in Microsoft Teams. (This isn't just Outlook email; it also includes Outlook.com, Gmail.com, and more.)
Guest access is included with all Office 365 Business Premium, Office 365 Enterprise, and Office 365 Education subscriptions. Guest access is a tenant-level setting in Teams and is turned off by default.