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:
use Cartalyst\Sentry\Sentry;
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);