Here's how this will work, according to Microsoft's blog post:
"Beginning today, anyone with an Azure Active Directory (Azure AD) account can be added as a guest in Teams. That means anyone with one of the more than 870 million user accounts--across Microsoft commercial cloud services and third-party Azure AD integrated apps--can be added as a guest in Teams. Later, we'll add the ability for anyone with a Microsoft Account (MSA) to be added as a guest in Teams. If the guest doesn't have an existing MSA, they will be directed to create a free account using their current corporate or consumer email address, such as Outlook.com or Gmail.com."
The MSA support is expected "in the next few weeks," a spokesperson said.
In Teams, guest accounts will be managed using Azure Active Directory B2B Collaboration, which Microsoft says will enable "enterprise-grade security," with support for features like conditional access.