1. Is Sheets Add-on free of cost?
Yes it's free of cost. It uses your API credits which you can upgrade later on.
2. How do I authenticate in the add-on?
Once you have installed the add-on, you are asked to authenticate your account by providing your API key which you can find in your account settings
3. How to find email addresses with the add-on?
Using domain search you can find email addresses using domain names. You can use this feature under the "Search" tab of the add-on
Using Email finder to find professional's email addresses from first and last names and domain names. It also tells you the validity of the emails if available. You can use this feature under the "Finder" tab.