Somehow, where you are using Sentry, you're not using its Facade, but the class itself. When you call a class through a Facade you're not really using statics, it's just looks like you are.
Do you have this:
In your code?
Ok, but if this line is working for you:
$user = $this->sentry->register(array( 'username' => e($data['username']), 'email' => e($data['email']), 'password' => e($data['password']) ));
So you already have it instantiated and you can surely do:
$adminGroup = $this->sentry->findGroupById(5);