Apple - Cannot seem to add internal testers with existing iTunes Connect accounts

You can use a "+" sign to add the same email to your desired iTunes Connect Account.

If the email is [email protected], you can add it like this: [email protected]

The system ignores the "+" sign and any character after that, it works like a charm but you have to use the altered email for the login.

After emailing Apple Support about this, I received an email saying this:

If a person's Apple ID is already associated with another iTunes Connect organization, the user needs to provide a distinct email address that can be associated with the new organization; iTunes Connect creates an additional Apple ID for the user.

I think it seems odd that Apple would require someone with an existing IC account to create another one just to be an admin on another user's organization, but this did work for me.

I used another one of my emails and created a new Apple ID when adding myself as another admin into the group. When I finally received a beta tester invite through that email, I was also able to install and use the app through my previous Apple ID.