How to invite a (sub)group of external testers to beta test through Testflight?

I have nine external testers listed for the Prerelease part of my new app. When I upload a new build, I would like to be able to send an invitation to only some of these testers, but clicking the 'Send invites' button under Builds automatically sends an invitation to all of them. Does anyone know a way around this? (i.e. short of deleting some external testers)

If you go to Prerelease>External Testers you can click the + next to Testers and choose to "Add Existing Testers" this allows you to choose specific people to invite to test the build.