External groups receiving notifications without build being available

Can anyone tell me if I have an option set wrong with my TestFlight setup.


Our external testers are receiveing TestFlight notifications that a new version is available for testing when we upload a build to our internal testers only.


I would expect that these external testers would only receive the notification once I've made the build available to the group.


Anyone shed any light on this one?


Cheers,

Steve