There is a system in beta testing from Amazon Web Services (AWS) (search engine nod) called Amazon Cognito.
It is described in a blog here:
1. Could this be used to create a group which FileMaker Server could use for external authentication purposes?
2. Will this new technology allow us to provide some sort of true Multi-Factor Authentication (MFA)?
To me, this looks like a way to provide External Authentication options to small groups quickly, easily and cheaply. Has anyone tried it yet?