In the Google Developer Console (https://console.developers.google.com/project) go to your project then go to Permissions, any Member that is added there (editor or reader) will be able to go to your project and run 'Test Install Flow'.
For the chrome webstore, go to your developer dashboard (https://chrome.google.com/webstore/developer/dashboard) and add a user as a tester in 'Edit your tester accounts' near the bottom. Then in your listing, set your Visibility options to 'Private' and 'Only trusted testers from your developer dashboard'.
Edit Note that in both cases, the user doesn't have to be in your domain.
LT