Bug in Google Play apps testing

I have several problems with google play apps testing.

I have a beta user group with 100 users.

In the process given from google, I must add all users to a group, and then, when user receive a mail, he click a blue button "Become a tester"

I have 2 problems in this step.

1. I don't have a way to know who clicked this button and who didn't click, so I really don't have control of who has the correct version
2. After clicking blue button, when user has 2 google accounts, it fails, even if the user choose the correct email. I was able to pass this step by deleting an account, download app, and re-add the account. It seems like a bug to me.

So basically, I have a lot of trouble with all this process.

Is there any way to get it work better, or should I just buy an external service?