I never have given access to my wifi, nor my Netro unit, to those who winterize my inground system.  They can generally do all they need from outside.  You do need to turn off your water supply to the sprinklers before they can clear the lines.

Hi Marc, 

You can use Netro's shared user function. Please let the shared user download the app and use his own email register an account with Netro. Then you can add his email as a shared user in your account. Please go settings->controller->manage users->add manager 

Thanks 

Regards, 

Netro Support