Internal Testers must be assigned the Admin, Technical, App Manager, Developer, or Marketer role in iTunes Connect, and you can only have 25 of them. You should review the iTunes Connect Roles. If you don’t trust someone with these privileges, they aren’t “Internal”, they’re “External”.
You can have many more External users, but apps need to go through the App Store review process before these users can test them:
Apps made available to external testers require a Beta App Review and must comply with the full App Store Review Guidelines before testing can begin. A review is required for new versions of your app that contain significant changes. Up to 10 apps can be tested at a time, internally or externally.