2👍
can I authenticate Azure AD users in my 3rd party website without them having preregistered my site as an allowable source by their admin? Is there any functionality/setting/service in Azure AD that allows that?
As a matter of fact this is a general authentication related question, but the answer is no. Based on my understanding, users need to configure their app as a multi-tenant application and have the admin/user of the other tenant give consent depending on what permissions are needed. I would like to suggest you to check https://msdn.microsoft.com/en-us/library/azure/dn132599.aspx for the details of how to add, update, or remove an application in Azure Active Directory (Azure AD), it shows the different types of applications that can be integrated with Azure AD as well.