Azure Active Directory B2B Collaboration

Azure AD B2B Collaboration is now Generally Available! Azure AD B2B collaboration capabilities enable any organization using Azure AD to work safely and securely with users from any other organization, with or without Azure AD.

You will find the documentation on the following site:
https://docs.microsoft.com/en-us/azure/active-directory/active-directory-b2b-what-is-azure-ad-b2b